Monoclonal antibody against myeloid leukemia cell line (KG-1)

A murine monoclonal antibody (WI-5) was produced against a myeloid leukemia cell line (KG-1). The antibody was immunoglobulin G3K. It reacted only against KG-1 cells and failed to react against 33 other cell lines representing fibroblasts, solid tumors, and cells of myeloid and lymphatic origin. It also showed no reaction against normal red blood cells, granulocytes, platelets, monocytes, and T- and B-lymphocytes. Similarly, there was no reaction against lymphocytes transformed by mitogens. Peripheral blood and bone marrow samples from acute granulocytic and acute lymphocytic leukemia, chronic granulocytic leukemia, chronic granulocytic leukemia in blastic crisis, and chronic lymphocytic leukemia failed to react with WI-5. It was suggested that WI-5 detected a unique antigen on KG-1 cells.     

Specificity of a monoclonal antibody to Drosophila melanogaster neoplastic hematopoietic cell surfaces as detected by immunofluorescence

We have generated, and characterized the immunological specificity of, a monoclonal antibody (MAb) termed 2B3G8 that recognizes cell-surface determinant(s) of D. melanogaster neoplastic hematopoietic tissue. This antibody was generated against antigens of a tumor line derived from neoplastic lymph glands, the hematopoietic organs, of a growth control mutant, Tumorous-lethal (Tum1). 2B3G8 does not recognize any of a panel of normal Drosophila cells or transformed cell lines from mouse or human sources as assayed by indirect immunofluorescence. Enzymatic digests did not remove this antigen from neoplastic blood cells or uncover the antigen in normal blood cells. Thus, we have generated a MAb that serves as a marker for the neoplastic hematopoietic condition in Drosophila which is a model for vertebrate neoplasia. This antibody is potentially useful for the study of oncodevelopmental antigens in developing hematopoietic systems and for detecting the onset of the neoplastic condition.     

A cell surface antigen (BAL) defined by a mouse monoclonal antibody inducing apoptosis in a human lymphocytic leukemia cell line

The lack of apoptosis or programmed cell death in human tumor cells has been suggested to be one factor allowing uncontrolled growth of neoplasms. We have developed a mouse monoclonal antibody (MAb) that induces programmed cell death in a human acute leukemia cell line (KM-3) of the pre B-cell type. Stable, antibody-producing hybridomas were produced by fusing mouse myeloma cells to spleen cells from mice immunized with viable KM-3 cells. Incubation of KM-3 cells with the MAb (designated anti-BAL) resulted in growth inhibition and subsequent cell death within 2-3 days. Anti-BAL required cross-linking with a rabbit anti-mouse antibody to induce DNA fragmentation typical of apoptosis. Immunoblotting experiments with anti-BAL identified a 37-kDa protein, apparently different from any previously described apoptosis-related surface antigen. Strongest expression of the antigen was generally found on cells of lymphoid or myeloid origin. However, several other cell types such as fibroblasts and endothelial cells were also stained by anti-BAL in flow cytometry but less intensively. Despite the apparent presence of this cell surface-bound 37-kDa antigen on several normal and malignant cell types, anti-BAL induced cell death only in human malignant cell lines expressing a more immature phenotype. © 1994 Wiley-Liss, Inc.     

Multidrug resistance in cultured human leukemia and lymphoma cell lines detected by a monoclonal antibody, MRK16

Forty cultured human leukemia and lymphoma cell lines never exposed to anticancer agents in culture, apart from doxorubicin (ADM)-resistant K562/ADM, were examined for reactivity with a monoclonal antibody, MRK16 in F(ab')2 form [MRK16-F(ab')2], which recognizes P-glycoprotein (P-gp). The relative resistance index to various drugs was calculated by dividing the 50% growth inhibitory concentration (IC50) of the test cell line by IC50 of K562, which was the negative control in the antibody experiment. MRK16-F(ab')2 reacted with four cell lines, K562/ADM, KYO-1, HEL and CMK, which had relative resistance index values of 2 or more to vincristine (VCR), vindesine, vinblastine, ADM, daunorubicin, mitoxantrone (MIT), etoposide (VP-16) and actinomycin-D (ACT-D). The level of resistance to VCR and ADM in these cell lines decreased significantly in the presence of 10 microM verapamil in vitro. Significant expression of mRNA of P-gp gene was also detected in K562/ADM, KYO-1 and HEL. MRK16-F(ab')2 did not react with 36 other cell lines. Among them, three cell lines, PL-21, P31/FUJ and KOPM-28, had relative resistance index values of 2 or more to anthracyclines, MIT and VP-16, but not to vinca alkaloids or ACT-D. The level of ADM-resistance in these cell lines did not decrease significantly in the presence of 10 microM verapamil. Five cell lines, ATL-1K, HL-60, KMOE-2, ML-1 and U266, had relative resistance index values of 2 or more to some of the drugs, but not to the others, and 19 other cell lines did not. These results indicate that the reactivity of MRK16-F(ab')2 correlates with a relative resistance index of 2 or more to all these drugs in cultured human leukemia and lymphoma cell lines.     

Expression of functional epidermal growth factor receptors in a human hematopoietic cell line.

To test the feasibility of using the human epidermal growth factor receptor (EGFR) as a model for growth factor receptor action in human hematopoietic cells, we infected Burkitt lymphoma cells (Namalwa) with a recombinant amphotrophic retrovirus containing a thymidine kinase promoter-driven human EGFR complementary DNA and the neomycin resistance gene. Neomycin-resistant cells expressing surface EGFR were selected by cell sorting using anti-EGFR monoclonal antibody 225. The selected cells expressed a Mr 170,000 protein immunoprecipitated by monoclonal antibody 225 and apparently identical to EGFR from A431 carcinoma cells. Infected Namalwa cells expressed 42,000 epidermal growth factor (EGF) binding sites/cell, and Scatchard analysis showed two affinities (Kd approximately 5 nM and approximately 0.5 nM). EGFR autophosphorylation was detected using antiphosphotyrosine antibodies after 5 min exposure to EGF. EGF binding induced rapid EGFR internalization (t1/2 = 9 min) and mobilization of transferrin receptors to the cell surface within 1 min. In fetal bovine serum-containing and serum-free cultures, EGF did not stimulate Namalwa cell proliferation. However, in the presence of 1.25% dimethyl sulfoxide (DMSO), EGF caused a dose-dependent increase in DNA synthesis. DMSO induced a cell cycle block in G1, which was partially reversed by EGF. DMSO induced changes in some B-cell markers suggesting cellular differentiation and increased surface EGF receptor number. Cells grown in DMSO and EGF were established as an EGF-dependent cell line for greater than 12 weeks, whereas cells grown in DMSO without EGF died within 1-2 weeks. Namalwa cells expressing EGFR demonstrated more rapid tumor growth in athymic mice. These studies demonstrate expression of functional EGFR mediating early biochemical and growth responses in a human hematopoietic cell, and indicate that EGFR can be used as an effective model in human hematopoietic cells. Results using DMSO are consistent with studies in other human leukemia cells indicating that agents inducing differentiation can restore growth factor dependence in previously factor-independent leukemia cells.     

Production of thymine glycols in DNA by N-hydroxy-2-naphthylamine as detected by a monoclonal antibody

We have quantitated the production of thymine glycols in DNA following treatment of cultured human fibroblasts or DNA in solution with the carcinogen N-hydroxy-2-naphthylamine. Thymine glycols, detected by using a monoclonal antibody specific to this base damage, were produced in DNA in a dose dependent manner both in vitro and in vivo. Exposure of DNA to N-hydroxy-2-naphthylamine in the presence of catalase and superoxide dismutase, which break down hydrogen peroxide and superoxide anions, respectively, inhibited the production of this base damage. Thymine glycols were efficiently removed from DNA in both normal human fibroblasts and in cells from a patient with xeroderma pigmentosum complementation group A, which are deficient in nucleotide excision repair.     

Immunohistochemistry of human gastrointestinal cancer-associated antigen detected by monoclonal antibody PA 8-15

A murine monoclonal antibody (MAb), PA 8-15, was produced against a newly established human pancreatic adenocarcinoma cell line, SUIT-2. With the avidin-biotin immunoperoxidase technique, PA 8-15 MAb reacted strongly with 27 of 28 formalin-fixed paraffin-embedded pancreatic adenocarcinoma tissues. All six gall bladder carcinomas and all nine biliary tract carcinomas also showed positive reactions. In addition, PA 8-15 MAb reacted with gastric carcinomas (6/10), colorectal carcinomas (7/11), and some other primary adenocarcinomas. The distribution of PA 8-15 antigen on the same tissues of pancreatic cancer was different from those of CA 19-9 and DU-PAN-2 antigens and CEA. PA 8-15 MAb stained only the epithelium of the pancreatic duct, gall bladder and bile duct in normal adult tissues, and some normal fetal glandular epithelial cells. However, PA 8-15 MAb was not reactive with inflammatory or benign tumors of the digestive system except for the epithelium, as was seen in normal adults. Reactivity of PA 8-15 MAb with tissue specimens largely disappeared after treatment with neuraminidase, while oxidation with periodate or trypsin digestion did not alter the staining intensity, indicating that antigenic determinants may be at least partly of sialylated carbohydrate nature. These results suggest that PA 8-15 MAb detects a new oncofetal antigen in gastrointestinal cancers, especially of the pancreatico-biliary tract.     

A human breast tissue-associated antigen detected by a monoclonal antibody

Two monoclonal antibodies were produced in mice immunized with the human breast carcinoma cell line MCF-7. One antibody (24-17.1) reacted with MCF-7 and other breast tumor cell lines and detected an antigen of Mr 95,000. This antigen was not breast-specific because other tumor cell lines were also reactive. The second antibody (24-17.2) detected an antigen of Mr 100,000 (initially appearing to be specific for breast tissue and possibly for breast carcinomas) which was present on 10 of 10 malignant breast lines and absent from 41 of 43 other cell lines of differing origins. The antigen could not be detected by absorption or a direct test on normal tissues (liver, kidney, heart, spleen) or on lymphocytes. In addition, the 24-17.2 antibody reacted absorptively with 12 of 13 fresh breast carcinoma samples but not with fresh colon carcinoma samples. The 100,000-Mr antigen detected by the 24-17.2 antibody appeared to be distinct from the other components of normal breast, such as casein, lactalbumin, or milk fat globulin protein. This evidence indicated that the 24-17.2 antibody detected a human breast carcinoma-associated antigen (HBCAA). However, further histologic studies were used to determine the cellular distribution of the HBCAA, which was found on malignant breast epithelium, the epithelium of gynecomastia, and in lesser amounts and differently distributed on normal breast epithelium. The antigen was also found in several other tissues; nonetheless, the anti-HBCAA could be detected in increased amounts in the sera of patients with breast cancer.     

Selective loss of expression of a tumor-associated antigen on a human leukemia cell line induced by treatment with monoclonal antibody and complement

The sensitivity of a common acute lymphoblastic leukemia-associated antigen (cALLa)-positive, human leukemia pre-B-cell line to killing by antibody and complement was studied. A stable subpopulation was selected by its ability to survive four sequential treatments with excess monoclonal antibody (MoAb) directed against an Mr 24,000 glycoprotein associated with human leukemia cells and excess rabbit complement. Analysis of the antigen expression by individual cells within the parental and the selected cell populations was achieved by flow cytometry and demonstrated a marked decrease of the leukemia-associated antigen expression on individual cells within the selected subpopulation. These low-antigen-density cells were stable in subculture, and the immunoglobulin heavy-chain gene rearrangement of the parent population and the low-antigen-density subpopulation were identical, indicating that they were derived from a single cell source. The selection of this subpopulation was specific in that the expression of a second antigen recognized by a cALLa-specific MoAb was not affected. The presence of subpopulations of tumor cells with low levels of surface antigen expression that are resistant to killing upon addition of excess antibody and complement will prove to be an obstacle to the use of this approach to eliminate tumor cells from bone marrow that is to be used for autologous transplantation.     

A human monoclonal antibody targeting the stem cell factor receptor (c-Kit) blocks tumor cell signaling and inhibits tumor growth

Stem cell factor receptor (c-Kit) exerts multiple biological effects on target cells upon binding its ligand stem cell factor (SCF). Aberrant activation of c-Kit results in dysregulated signaling and is implicated in the pathogenesis of numerous cancers. The development of more specific and effective c-Kit therapies is warranted given its essential role in tumorigenesis. In this study, we describe the biological properties of CK6, a fully human IgG1 monoclonal antibody against the extracellular region of human c-Kit. CK6 specifically binds c-Kit receptor with high affinity (EC₅₀ = 0.06 nM) and strongly blocks its interaction with SCF (IC₅₀ = 0.41 nM) in solid phase assays. Flow cytometry shows CK6 binding to c-Kit on the cell surface of human small cell lung carcinoma (SCLC), melanoma, and leukemia tumor cell lines. Furthermore, exposure to CK6 inhibits SCF stimulation of c-Kit tyrosine kinase activity and downstream signaling pathways such as mitogen-activated protein kinase (MAPK) and protein kinase B (AKT), in addition to reducing tumor cell line growth in vitro. CK6 treatment significantly decreases human xenograft tumor growth in NCI-H526 SCLC (T/C% = 57) and Malme-3M melanoma (T/C% = 58) models in vivo. The combination of CK6 with standard of care chemotherapy agents, cisplatin and etoposide for SCLC or dacarbazine for melanoma, more potently reduces tumor growth (SCLC T/C% = 24, melanoma T/C% = 38) compared with CK6 or chemotherapy alone. In summary, our results demonstrate that CK6 is a c-Kit antagonist antibody with tumor growth neutralizing properties and are highly suggestive of potential therapeutic application in treating human malignancies harboring c-Kit receptor.     

Production by undifferentiated myeloid leukemia cells of a novel growth-inhibitory factor(s) for partially differentiated myeloid leukemic cells

Mouse monocytic Mm-A cell line is a highly leukemogenic variant cell line of the monocytic and non-leukemogenic Mm-1 cell line, which developed spontaneously from mouse myeloid leukemia M1 cells. Growth-inhibitory factor (GI factor) for Mm-A cells was found in conditioned medium (CM) of differentiation inducer-resistant myeloblastic M1 cells (clone R-1). The R-1 cells were cultured with or without 2% calf serum for 2 days, and the CM was fractionated with 50% ammonium sulfate and used as the GI factor preparation (termed R1CM). When Mm-A cells were cultured with 5% (v/v) R1CM for 3 days, their growth was inhibited about 80%. This inhibition of Mm-A cell growth by R1CM was irreversible. This GI factor also inhibited the growth of M1 cells that had been pretreated with inducer and had expressed some differentiation-associated properties but still retained a proliferative capacity. In contrast, it scarcely inhibited the growth of untreated M1 cells. The GI factor inhibited the growth of other mouse monomyeloblastic leukemic WEHI-3B D+ cells pretreated with a differentiation inducer, retinoic acid, and mouse monocytic leukemia J774.1 cells. However, it did not affect the growth of human monocytic (U937 and THP-1) or myeloid (KG-1, ML-1, and HL-60) cell lines. These results suggest that GI factor produced by parent myeloblastic and inducer-resistant M1 cells preferentially inhibits the growth of mouse monocytic leukemia cells in intermediate stages of differentiation from myeloblastic leukemia cells to mature macrophages.     

Antitumour reactions of monoclonal antibody against a human osteogenic-sarcoma cell line.

Monoclonal antibody against an osteogenic-sarcoma cell line (791T) was prepared by production and cloning of a somatic-cell hybrid between the mouse myeloma P3-NS1 and spleen cells from 791T-immunized mice. Three clones of hybridoma producing antibody against 791T, as detected by 125I-labelled Protein A binding, were tested against a range of normal and tumour cell targets to determine the pattern of expression of the antigen detected. The 3 clones had identical activity. They reacted strongly against 791T cells and another osteogenic sarcoma, 788T, and more weakly against a further 2 from a total panel of 10 osteogenic-sarcoma lines. The antibody was negative for fibroblasts from the donor of 791T, and for other fibroblasts, human red blood cells, human peripheral mononuclear cells and sheep red blood cells. When tested against a panel of unrelated tumours, they reacted against individual cell lines derived from carcinomas of colon, lung, bladder and cervix. These cross-reactions were not observed with other colon or lung carcinomas, and it is suggested that the antibody was reacting with a tumour-associated antigen expressed randomly on different tumour types, rather than specifically on osteogenic sarcomas.     

Characterization of a new myeloid leukemia cell line with normal cytogenetics (CG-SH)

A new myeloid leukemia cell line (CG-SH) with normal cytogenetics was established from a patient with acute myelogenous leukemia (AML) following myelodysplastic syndrome (MDS). The cells of CG-SH are immature blasts and have an immature myeloid phenotype (positive for myeloperoxidase, CD7, CD34, CD38, CD117, HLA-DR, negative for CD10, CD19, CD20, CD41, CD42). A partial expression of CD13, CD15, CD65 and a weak expression of CD33 and CD133 was noted. The cells are negative for EBER. By molecular analysis, a mutation of NRAS and heterozygous mutations of RUNX1 were detected. No mutations were detected in FLT3-ITD, MLL-PTD or NPM1. By real-time PCR, a series of 19 microRNAs was identified which are strongly expressed in CG-SH. In conclusion, a new cell line was established which will be useful for the study of AML with normal cytogenetics and mutations in NRAS and/or RUNX1.     

Epidermal growth factor receptor expression in human lung carcinomas defined by a monoclonal antibody

Acetone-fixed frozen tissue sections from 56 cases of human lung carcinoma were tested for reactivity by an indirect immunoperoxidase technique with a monoclonal antibody (MAb 528) specific for the external domain of the epidermal growth factor receptor (EGFR). MAb 528 reacted with all epidermoid (22/22) and large-cell (4/4) lung carcinomas evaluated. The antibody was also positive with a subset of lung adenocarcinomas (13/21) and did not react with small-cell lung cancers (SCLCs) (0/9). MAb 528 also stained normal bronchial epithelium identified within the tumor sections of 5 cases. Thus EGFR was expressed by all epidermoid and large-cell lung carcinomas examined, a subset of lung adenocarcinomas, and normal bronchial epithelium. EGFR expression was not identified in any of the SCLCs tested. These data imply that immunohistochemical detection of EGFR expression may find future application in distinguishing epidermoid, large-cell, and some adenocarcinomas of the lung from SCLCs.     

Notch signaling inhibits the growth of the human chronic myeloid leukemia cell line K562

Notch signaling functions in the development of some types of leukemia and lymphoma, but the relationship between Notch signaling and chronic myeloid leukemia (CML) remains to be elucidated. In this study, we examined the expression of Notch receptors and ligands in the human CML cell line K562. When the active form of Notch1, the Notch intra-cellular domain (NIC), was over-expressed in K562, the proliferation of K562 was mildly but significantly inhibited, accompanied by increased Hes1 mRNA level. On the other hand, when Notch signaling was attenuated by over-expression of a dominant-negative RBP-J, RBP-J(R218H), in K562 cells, the proliferation of K562 was increased. Moreover, we found that activation of Notch signaling inhibited while repression of Notch signaling promoted the colony-forming activity of K562 cells. We examined cell cycle-related molecules in K562 transfected with NIC or RBP-J(R218H), and found that the protein level of the retinoblastoma gene product (the Rb protein) was induced in K562 expressing NIC, and down-regulated in K562 expressing RBP-J(R218H). These data suggest that the Notch signaling may function as a tumor inhibitor in human CML cells.     

Growth-associated shedding of a tumor antigen (CE7) detected by a monoclonal antibody

A monoclonal antibody was developed against an antigen, termed CE7, which was highly expressed on the surface of rat fibrosarcoma KMT-17 cells (clone A3) cultured in low serum medium (A3-1% FCS). The CE7 antigen was not detectable on A3 cells cultured in ordinary high serum medium (A3-10% FCS), on in vivo passaged A3 cells, or on parental in vivo KMT-17 cell line. However, immunoelectron microscopy and Western blot analyses indicated that CE7 antigen was produced by these tumor cells in all circumstances but was shed from their surfaces in vesicular form into the surrounding tissue culture medium or ascites, unless low serum concentration prevailed and disappeared from their cell surfaces. We have previously reported that the immunogenicity of A3 cells was increased when the serum concentration was lowered from 10% to 1% and the phenomenon paralleled the CE7 antigen expression on the A3 cells. These results suggest that the CE7 antigen could be a tumor-associated rejection antigen and that the expression of the CE7 antigen on A3-1% FCS cells (which is shed by high serum culture or in vivo transplantation and disappears from the cell surface) may play a role in immunological responses against the tumor cells.     

NHL-30.5: a monoclonal antibody reactive with an acute myeloid leukemia (AML)-associated antigen

The reactivity of a murine IgG1 monoclonal antibody, NHL-30.5, that detects a surface antigen expressed on acute myeloid leukemia (AML) cells has been studied. Initially raised against the HL-60 cell line, NHL-30.5 has subsequently reacted with blood and/or bone marrow cells from 15 of 19 AML patients studied at presentation or in relapse, 1 patient with chronic myelomonocytic leukemia (CMML), 1 patient with myelofibrosis (MF) who subsequently developed AML, and 1 of 5 patients with acute lymphoblastic leukemia (ALL). It has shown no detectable binding to cells from AML patients in remission (0/3), patients with chronic myelogenous leukemia in chronic phase (CML) (0/7), normal bone marrow (0/9), normal peripheral blood mononuclear cells, granulocytes, platelets, erythrocytes, monocytes, or splenocytes by radioimmunoassay or fluorescence analysis using flow cytometry. HL-60 cells induced to differentiate following incubation in the presence of dimethylsulfoxide (DMSO) lost their ability to bind NHL-30.5. Immunoprecipitation of iodinated HL-60 cell surface components showed the antigen to have an apparent mol./wt of 180,000 under reducing conditions. These results suggest that the antigen is different from any other myeloid antigens reported to date, and may be useful in further studies of leukemic cell phenotypes.     

Mechanisms of growth factor expression in acute myeloid leukemia (AML)

To investigate possible mechanisms of growth factor expression in acute myeloid leukemia, genes for granulocyte macrophage colony-stimulating factor (GM-CSF) were analyzed by Southern blots in 20 patients, for M-CSF in 13, for interleukin-6 (IL-6) in 14, for IL-6 receptor in 14 and for G-CSF in five patients. Only in one patient a complex rearrangement of the G-CSF gene with possible amplification was noted indicating rarity of direct alterations of growth factor genes in acute myelogenous leukemia (AML). Spontaneous m-RNA expression for GM-CSF was found in only one of 20 patients, and for IL-6 in eight of 11 patients. In vitro incubation of AML cells of eight patients with recombinant tumor necrosis factor for 24 hr revealed induction of GM-CSF m-RNA expression in three cases and GM-CSF protein expression in two of them. These data suggest that spontaneous GM-CSF production occurs rarely in AML and that monokines, such as tumor necrosis factor, may induce GM-CSF in AML cells. Therefore, interactions of AML cells with normal or malignant accessory cells may be important for autocrine stimulation in AML. Our data suggest that ectopic growth factor secretion is not the primary cause of generating AML but may contribute to progression of the disease. Alternatively, AML may represent a heterogenous group of leukemias with different etiology but similar phenotype.     

Production and characterization of a monoclonal antibody to a myeloid specific differentiation antigen

A monoclonal antibody termed NC-1 was produced which binds to an antigen present on the human promyelocytic leukemia cell line HL-60 and peripheral blood neutrophils. The antibody bound to the majority of promyelocytes in the HL-60 culture, but not to myeloblasts. No antibody reactivity was detected to a range of other cell lines or to a limited number of normal human tissues. Peripheral blood lymphocytes, monocytes, basophils, eosinophils, erythrocytes and platelets did not react with the antibody. Bone marrow smears exhibited binding of NC-1 to myeloid cells at the promyelocyte and later stages of differentiation along the granulocyte lineage. The KG-1 myeloblastoid and U937 myelomonocytic lines could be induced to express the antigen, when exposed to neuraminidase which removes terminal sialic acid from carbohydrate residues. Similarly myeloblasts in bone marrow samples bound NC-1 when they were treated with neuraminidase. HL-60 cells induced to differentiate to granulocytes by treatment with retinoic acid continued to express the antigen. A similar result was observed when HL-60 cells were induced to differentiate to monocytes, even though blood monocytes failed to bind the antibody. These data indicate that the antibody NC-1 reacts with an antigen expressed on myeloid cells beyond the promyelocyte stage of differentiation.     

Distribution of a new B-cell-associated surface antigen (BL7) detected by a monoclonal antibody in human leukemic disorders

A murine monoclonal antibody (anti-BL7) was raised by immunization of BALB/c mice with a precursor B-cell line (Josh-7) which detects a heat-stable, nonimmunoprecipitable antigen. The expression of BL7 was investigated in peripheral blood and/or bone marrow leukemic cell suspensions stained by indirect immunofluorescence and analyzed by flow cytometry. Lymphoblasts from 43 of 43 cases of "null" acute lymphoblastic leukemia were BL7-. Five cases of T-acute lymphoblastic leukemia and 5 cases of terminal deoxynucleotidyl transferase-positive blastic chronic myelogenous leukemia were also BL7-. All 63 cases of B-cell chronic lymphocytic leukemia were BL7+. Neoplastic cells in 22 of 28 cases of B-cell non-Hodgkin's lymphomas in leukemic phase were also BL7+. Expression of BL7 showed some correlation with Rappaport's histological classification. Four cases of multiple myeloma and plasma cell leukemia were BL7-. Twenty-three cases of acute nonlymphocytic leukemias were also analyzed. Of these, only the acute promyelocytic (M3,4 cases) and acute myelomonocytic (M4, one case) varieties expressed BL7 on a small proportion (approximately 15%) of the leukemic cells. All other subgroups were BL7-. The reactivity of anti-BL7 was compared to other B-cell antibodies on selected samples and was shown to be different from B1, B2, and the BA antibodies. Anti-BL7 is a unique monoclonal antibody useful in the study of B-cell cancers.