In Vivo Enzyme Control Through a Strong Stationary Magnetic Field—the Case of Thymidine Kinase in Mouse Bone Marrow Cells

Summary

The influence of a strong homogeneous and stationary magnetic field (SMF) on the activity of the enzyme thymidine kinase (TdR-K) in bone marrow cells, and as a consequence of this on the incorporation of ¹²⁵I-labelled 5-iodo-2-deoxyuridine (¹²⁵IUdR) into DNA of mice and into isolated bone marrow cells in vitro, was assayed after exposure of immobilized mice. No effect could be elicited in moving mice, in cells in suspension or in enzyme in solution.

The response depended on the body temperature during exposure: at 27°C and 29°C there was an increase and at 37°C and 39·5°C a depression of enzyme activity. The TdR-K activity at low temperature increased with the field strength ranging from 0·2 to 1·4 T. Thirty minutes were required for full expression of the effect at 1·4 T; 5–10 min were needed after exposure for a return to base-line levels.

Mice were given total-body irradiation at a dose of 0·1 Gy ¹³⁷Cs gamma rays and then exposed immediately to a magnetic field at 1·4 T for 30 min at a body temperature of 27°C; gamma irradiation no longer inhibited the enzyme. Exposure to the magnetic field further removed from the time of gamma irradiation, did not negate the inhibitory effect of gamma irradiation.

The observed responses to given challenges in this complex system support the hypothesis that the magnetic field affects TdR-K activity by way of a mediating structure, such as a membrane.     

3H-TdR与125I-UdR掺入淋巴细胞增殖的比较

目的 比较 ³H-TdR与 ¹²⁵I-UdR掺入淋巴细胞的增殖效应。方法 用 ³H-TdR与 ¹²⁵I-UdR掺入法测定淋巴细胞和Daudi淋巴瘤细胞的增殖效应。结果 ³H-TdR和 ¹²⁵I-UdR在正常淋巴细胞中的掺入率分别为20.95%±1.06%和1.00%±0.04%,在Daudi淋巴瘤细胞中的掺入率分别为29.94%±4.10%和6.02%±0.73%。 ³H-TdR在细胞中的掺入率明显高于 ¹²⁵I-UdR;且 ³H-TdR和 ¹²⁵I-UdR在淋巴瘤细胞中的掺入率高于正常淋巴细胞。结论 就淋巴细胞而言,作为示踪剂 ¹²⁵I-UdR不能替代 ³H-TdR;但对于淋巴瘤细胞,能否代替 ³H-TdR有待于进一步研究。     

Egr-IFNγ基因治疗联合125I-脱氧尿嘧啶核苷治疗抑瘤效应的实验研究

目的 探讨Egr-IFNγ基因治疗联合放射性核素 ¹²⁵I -脱氧尿嘧啶核苷治疗方案在荷H22肝癌细胞小鼠体内抑瘤效应及机制。方法 小鼠肿瘤局部注射脂质体包裹的质粒,注射后48 h,肿瘤局部注射370kBq ¹²⁵I -UdR。观察各组小鼠治疗后不同时间肿瘤生长率;治疗后第3 天,检测肿瘤胞浆蛋白中IFNγ的表达和脾脏CTL细胞毒活性。结果 基因-放射核素治疗后第6～15天,pcDNAEgr-IFNγ+ ¹²⁵I -UdR组肿瘤生长率明显低于对照组、 ¹²⁵I -UdR组及pcDNAEgr-1+ ¹²⁵I -UdR 组;基因-放射性核素治疗后第3天,pcDNAEgr-IFNγ+ ¹²⁵I -UdR组肿瘤胞浆蛋白中可检测到IFNγ的表达,其余组肿瘤胞浆蛋白中未检测到IFNγ的表达;pcDNAEgr-IFNγ+ ¹²⁵I -UdR组小鼠脾脏CTL细胞毒活性明显高于其余组 (P<0.01)。结论 pcDNAEgr-IFNγ基因治疗联合放射性核素 ¹²⁵I -UdR治疗抑瘤效应明显优于单纯 ¹²⁵I -UdR放射性核素治疗。     

Reutilization of 125I-UdR during growth of a solid mammary carcinoma: implications for the 125I-UdR loss technique

Reutilization of thymidine (TdR) and 5-iodo-2'-deoxyuridine (I-UdR) released by dying tumour cells was assayed in the syngeneic adenocarcinoma EO 771 by injecting heat killed, labelled tumour cells into tumours. 3H and 125I liberation from labelled breakdown products was measured in tumours of various sizes without or with separation of tumours into viable and necrotic portions. Internal reutilization of 3H-TdR was considerably greater than that of 125I-UdR. 125I-UdR released by dying tumour cells was reutilized at about 10%. There was no significant increase in 125I-UdR reutilization during tumour growth. It is concluded that measurements of radioactivity loss by the 125I-UdR technique can result in underestimating the real cell loss depending on the amount of internal reutilization by the tumours investigated. Compared with 3H-TdR 125I-UdR is the tracer of choice for long term studies of cell loss.     

Cell loss from viable and necrotic tumor regions after local gamma irradiation measured by 125I-UdR

Using a tracer technique, loss of cells from perivascular and average tumor cells of the syngeneic mammary adenocarcinoma EO 771 in male C57Bl/6J mice may be measured in the living animal, by the use of 125-labelled 5-iodo-2'-deoxyuridine (125I-UdR). It was the purpose of this paper to compare measurements in vivo with those made in vitro following local 60Co-gamma irradiation in the absorbed dose range from 10 to 27.5 Gy, incorporation of radioactivity into DNA of tumor cells and activity loss from labelled tumor cells were measured externally by a special scintillation counter device. In addition, by injecting the vital dye "light green" into the mice the I-125-activity of the stained viable and unstained necrotic regions were separately measured for loss of activity following gamma irradiation. A comparison was made between radiation induced growth delay and the depression of 125I-UdR incorporation into DNA of the proliferating tumor cells. After local tumor irradiation with a dose of 27.5 Gy 60Co gamma rays an enhancement of the activity loss by 0.5% per hour was externally observed for the perivascular tumor cell population. A lower enhancement of 0.4% per hour was externally registered in the average tumor cell population. Both values were evaluated relative to sham-irradiated control tumors. The measurements on isolated tumors were in comparatively good agreement with the external values. The activity loss rate from the viable, euoxic tissue increased by 0.4% per hour after 27.5 Gy 60Co gamma rays and by 0.3% per hour in the average cell population, the latter representing a mixture of euoxic and hypoxic cells. The results demonstrate, that the external measurements are a good indicator for radiation effects under in vivo-conditions.     

125I-脱氧尿嘧啶核苷对淋巴瘤细胞Raji和Daudi的杀伤作用

目的 探讨¹²⁵I-脱氧尿嘧啶核苷(¹²⁵I-UdR)在淋巴瘤细胞Raji和Daudi中的特异性摄取及其杀伤效应。方法 测量Raji、Daudi细胞和细胞核在含不同放射性浓度¹²⁵I-UdR的培养液中培养不同时间后的活度;用噻唑蓝(MTT)实验和碘化丙啶(PI)染色周期分析评价¹²⁵I-UdR对Raji和Daudi细胞的杀伤作用。结果 Raji和Daudi细胞摄取¹²⁵I-UdR的量明显高于Na¹²⁵I对照组(P<0.05),在100kBq/ml浓度时,Raji和Daudi细胞摄取¹²⁵I-UdR的量分别为(14 414±95)和(6916±53.69)Bq/10⁶细胞,而对Na¹²⁵I的摄取量分别为(68±3.8)和(324±32.8) Bq/10⁶细胞;细胞和细胞核中¹²⁵I-UdR的量随培养基中¹²⁵I-UdR放射性浓度以及培养时间的增加而增加;¹²⁵I-UdR组的存活分数明显低于Na¹²⁵I对照组(P<0.05),以500kBq/ml浓度培养48h时, Raji和Daudi细胞¹²⁵I-UdR组的存活分数分别为(19.78±1.39)%和(43.17±2.69)%,而Na¹²⁵I组的存活分数分别为(79.10±1.79)%和(80.36±6.12)%;细胞存活分数有随培养基中放射性浓度增加而降低的趋势。结论 ¹²⁵I-UdR可被Raji和Daudi细胞特异性摄取并进入细胞核中,进而杀死细胞,其作用具有明显的时间-效应和剂量-效应关系。     

^125I-脱氧尿苷治疗膀胱癌的实验与临床研究

放射性核素通过电子俘获和（或）内转换衰变产生低能电子（〈1 keV）的俄歇效应,掺入细胞DNA后具有显著的细胞毒性。^125I-脱氧尿苷（^125I-UdR）是将^125I引入细胞核的有效载体,能特异性掺入S期细胞DNA。一系列研究表明,^125I能更多地被肿瘤细胞吸收,而不是正常的分裂细胞,从而有效地治疗恶性病变。由于膀胱为一天然囊腔,具有独特的易灌注及观察性,膀胱灌注^125I-UdR治疗膀胱癌,能高效、选择性杀伤肿瘤细胞,明显降低膀胱癌复发率,故可作为外科手术的辅助治疗手段,有希望成为一种安全、高效、不良反应小的治疗膀胱癌的新疗法。     

Effect of blood serum from irradiated mice on the incorporation of DNA, RNA and protein precursor in L929 cells. A sensitive assay for measuring low dose response

Serum from whole-body irradiated mice inhibits incorporation of DNA precursors into DNA of L929 cells in culture in a dose-dependent way. The humoral factor interfering with the incorporation of 3H-thymidine and 125I-iododeoxyuridine is identical to thymidine. The degree of depression of 125I-iododeoxyuridine-uptake is more sensitive than that of 3H-thymidine. Irradiation of donor mice does not confer a toxic effect of blood serum on cell growth in culture. Incorporation of 3H-leuchine into protein and 3H-cytidine into DNA and RNA is not affected by the serum of irradiated mice; there is no effect on the incorporation of 3H-cytidine from the intracellular precursor pool into DNA or RNA either. The present findings demonstrate the specificity and high sensitivity of the assay system for measuring thymidine concentration in mouse blood serum and point to possible applications of analysing abnormalities in DNA metabolism resulting in, or from, disturbances of the thymidine reutilization pathway.     

Vitamin E regulates changes in tissue antioxidants induced by fish oil and acute exercise

PURPOSE: Prooxidant effects of fish oil supplementation could unfavorably affect the cardiovascular benefits of fish oil. We tested the effects of 8 wk vitamin E cosupplementation with fish oil on antioxidant defenses at rest and in response to exhaustive exercise in rats. METHODS: Rats (N = 80) were divided into fish oil, fish oil and vitamin E (FOVE), soy oil, and soy oil and vitamin E (SOVE) supplemented groups. For the vitamin E supplemented rats, corresponding groups (FOVE-Ex and SOVE-Ex) performed an acute bout of exhaustive exercise after the supplementation period. RESULTS: Fish oil supplementation increased the activity of catalase, glutathione peroxidase, and glutathione-S-transferase in the liver and red gastrocnemius (RG) muscle. Fish oil decreased liver total glutathione (TGSH) levels. Vitamin E supplementation decreased antioxidant enzyme activities to levels at or near those in SOVE in a tissue specific pattern. Vitamin E increased TGSH in liver, heart, and RG. Regression analysis showed TGSH to be a negative determinant of protein oxidative damage as measured by protein carbonyl levels in both liver and RG. Catalase activity was associated with liver lipid peroxidation as measured by thiobarbituric acid-reacting substances. The exercise-induced decrease in hepatic TGSH tended to be less in FOVE versus SOVE. Exhaustive exercise also modulated tissue antioxidant enzymes. CONCLUSIONS: Vitamin E supplementation markedly decreased fish oil induced antioxidant enzyme activities in all tissues. Sparing of glutathione may be an important mechanism by which vitamin E decreased tissue protein oxidative damage.     

Prospective 153Sm-EDTMP therapy dosimetry by whole-body scintigraphy.

Samarium-153 ethylenediaminetetramethylene phosphonic acid (153Sm-EDTMP) effectively palliates painful bony metastases, but the standard recommended administered activity of 38 MBq.kg-1 may lead to significant myelotoxicity. Prospective individual dosimetry by urine collection and counting allow the bone marrow radiation dose to be limited to 2 Gy. Our novel whole-body scintigraphic method for prospective dosimetry was compared with the 5 h urine collection technique in 10 patients with bone metastases. Anterior and posterior whole-body images were obtained using identical acquisition parameters 10 min and 5 h after the intravenous injection of 740 MBq 153Sm-EDTMP. Total counts in each imaging study were corrected for background activity and time of injection and the bone activity at 5 h was determined. Bone activity was also calculated from a complete urine collection over 5 h, and these two values were compared. MIRD formulae were applied to calculate the radiation absorbed dose to the bone marrow from the injected activity. The total activity delivering a dose of 2 Gy to the bone marrow was then determined and constituted the amount given for therapy. Values for bone activity determined by imaging and by urine counting were concordant in all patients (correlation coefficient = 0.98). The total administered activity of 153Sm-EDTMP predicted on a 2 Gy bone marrow dose varied between 35 and 63% of the standard recommended regimen of 37 MBq.kg-1 and pain relief was experienced by eight of the ten patients. Administration of 153Sm-EDTMP according to the supplier's recommendations would have delivered bone marrow doses of 3.27-5.90 Gy in our patients, doses at which myelotoxicity would have been anticipated.     

中药对大鼠辐射损伤防护作用的实验研究

目的探讨中药当归、川芎、黄芪、丹参提取液对大鼠放射性骨髓损伤的防护作用.方法雄性SD大鼠60只,随机分为正常对照组、模型组、中药组,每组6只,在相同条件下饲养2周,模型组和中药组大鼠给予6.0 Gy 60Co γ射线一次性全身照射,继续饲养1周处死全部动物并取材.用骨髓细胞计数法计数骨髓有核细胞数,用Western blot法测定血管内皮生长因子（vascularendothelialgrowth factor,VEGF）、血小板衍生生长因子（ptatelet derived growth factor,PDGF）的蛋白含量.结果与正常对照组比较,模型组大鼠的骨髓有核细胞数、VEGF、PDGF蛋白含量明显减少（P＜0.01）,与模型组比较,中药组大鼠的骨髓有核细胞数、VEGF、PDGF蛋白含量明显增加（P＜0.01或P＜0.05）.结论中药当归、黄芪、川芎、丹参对大鼠放射性骨髓损伤有一定防护作用.     

Metabolic disease in animals

Rickets is a metabolic bone disorder characterized by osteopenic changes resulting from the failure of calcification of the osteoid matrix and absent mineralization of hypertrophic cartilage cells at the epiphyseal growth plates in growing primates, herbivores, swine, carnivores, and birds. The causes of rickets include inadequate dietary provision of calcium, phosphorus, and vitamin D. Osteomalacia in reptiles, simian bone disease in nonhuman primates, and osteodystrophia fibrosa (secondary hyperparathyroidism) or "bran disease" in herbivores are caused by a diet that has a much higher content of phosphorus than calcium, combined with inadequate exposure to direct sunlight. Medullary bone consists of interconnected spicules of bone resembling embryonic bone and is established in relation to the shell formation cycle of laying birds. Hypertrophic osteodystrophy develops in large-breed growing dogs, chickens, and guinea pigs and is possibly caused by vitamin C deficiency. Tibial dyschondroplasia is a defect in endochondral ossification characterized by a widened proximal tibial physis that is not penetrated by metaphyseal vascular sprouts, commonly found in growing broiler chickens, turkeys, and exotic birds.     

Effect of 24,25-dihydroxyvitamin D3 on precursor cells of osteogenesis in immobilized rats

The experiments were carried out on Wistar SPF rats that were immobilized for 35 days. By heterotopic marrow cell transplantation under the kidney capsule to the normal rats and by cloning these cells in vitro it was found that osteogenetic potentials were significantly inhibited and the amount of osteogenetic precursor cells was reduced. The addition of 24,25(OH)2D3 vitamin (at a dose of 1.25 micrograms per day) to the animal diet led to the normalization of the above parameters. It is assumed that immobilization-associated osteoporosis develops via, among other mechanisms, inhibition of histogenesis of stromal precursor cells. The beneficial role of vitamin D3 is actually the activation of histogenesis of these cells which results in the recovery of bone remodelling during immobilization.     

Radioprotection of whole-body gamma-irradiation-induced alteration in some haematological parameters by cysteine, vitamin E and their combination in rats

Radioprotective effect of cysteine, vitamin E and their combination on gamma-irradiation-induced alteration in some haematological parameters in male rats has been studied 24 and 48 hrs after whole-body gamma-irradiation at a dose level of 7.5 Gy. The results of this study reveal that gamma-irradiation caused a significant decrease in red blood cells (RBCs) count with insignificant change in hemoglobin level, 24 and 48 hrs postirradiation, gamma-irradiated rats showed as well a progressive decrease in their blood ATP, and serum-SH levels with a significant increase in blood glutathione (GSH) level. Administration of cysteine or vitamin E preceding gamma-radiation exposure gave a significant radioprotection to the above haematological parameters. However, combination of both agents afforded a better protection, so that most of the measured parameters were restored to the pre-irradiated values. Finally, the date demonstrate that the radioprotection provided by combined administration of vitamin E and cysteine is feasible and perhaps, even more efficient against radiation injury to RBCs. This will appreciate the usage of such combination in protecting the patient during radiotherapy.     

Decorporating efficacy of catecholaminocarboxylate chelating agents for thorium-234 and protective effects on associated radiation injury

The aim was to identify the decorporation and anti-oxidation efficacy of prompt and delayed consecutive administration of the catecholicpolyaminopolycarboxylate ligands 9501 and 7601 for radiothorium in vivo. The chelating agents 9501 or 7601 were administered intramuscularly to ICR mice 3 min or 3 days after intraperitoneal injection of 30 MBq kg(-1) (234)Th-citrate for 3 consecutive days. The animals were killed 4 or 5 days after administration of the chelating agents, respectively. The (234)Th radioactivity in the whole-body and its retention in liver and skeleton were determined. Malondialdehyde (MDA) production, as an index of (234)Th-induced lipid peroxidation in bone marrow and liver, was assayed and the number of bone marrow nucleated cells (NBMNC) were counted. The pathological changes of bone marrow and liver tissue were observed. CaNa(3)-diethylenetriaminepentaacetate (DTPA) and vitamin E were used as controls. The competitive ability of 9501 and 7601 to mobilize thorium with bovine serum albumin (BSA) was studied. Their inhibitory effect on superoxide anion radicals was measured by electron spin resonance. When promptly injected, 9501 or 7601 were superior to CaNa(3)-DTPA for reducing (234)Th retention in mouse. Their different bioactivity for decorporation of (234)Th was consistent with their competitive ability to mobilize thorium with BSA. Although the removal effectiveness of 9501 and 7601, given by delayed injection, was lower than that of the prompt administration, they could inhibit (234)Th-induced lipid peroxidation. This caused significant reductions of MDA content in bone marrow and liver and markedly ameliorated histological changes to bone marrow and liver tissue in (234)Th-treated mice. Their protective effects were better than CaNa(3)-DTPA and vitamin E. 9501 and 7601 could directly scavenge O(.-)(2). Their effects as O(.-)(2) scavengers were very significant. The chelating agents 9501 and 7601 are able to remove thorium as effectively as other commonly used agents like CaNa(3)-DTPA and reduce radiation dose. In addition, these agents have anti-oxidative action reducing both cell killing in the bone marrow and malondialdehyde levels, a measure of lipid peroxidation, in the bone marrow and liver. The protection from internally deposited radioactive material was predictable by the chelators' competitive ability to chelate thorium from BSA and their ability to act as an oxygen free-radical scavenger. The duel effect of reducing radiation dose and response could be important in reducing the risk from internally deposited gamma radionuclides.     

全身MRI与PET/CT在淋巴瘤骨髓浸润诊断及预后中的作用

淋巴瘤是一种血液系统恶性肿瘤。淋巴瘤骨髓浸润(BMI)使疾病分期上升至IV期, 是疾病进展、预后较差的标志。常规部位的骨髓活检(BMB)具有创伤性, 且检出率低。PET/CT与全身MRI的出现, 丰富了BMI的检测手段。PET/CT与全身MRI对于淋巴瘤, 尤其是侵袭性淋巴瘤BMI均具有较高的检出率, 二者孰高孰低, 尚未定论。对于红骨髓、良性骨髓病变(炎症等)、淋巴瘤BMI病灶以及肿瘤治疗后骨髓的变化与骨髓残留或复发病灶, 全身MRI很难区分, 而PET/CT却可以很好地鉴别这些病灶。但是, PET/CT存在电离辐射; 对于惰性淋巴瘤的BMI, 超出PET/CT分辨率的病灶, 可能出现假阴性; 某些情况会限制PET/CT的使用, 包括18F-FDG生理性摄取量可能发生改变的正常组织、18F-FDG摄取相关性炎症、高血糖或高胰岛素血症导致的18F-FDG分布的改变、肿瘤患者治疗后出现的骨髓活化等。然而, 这些情况可以使用全身MRI。因此, 全身MRI和PET/CT相辅相成, 优势互补, 但二者均不能代替BMB。对于常规BMB阴性, 但影像学提示阳性的患者, 在影像学引导下进行BMB, 可以提高BMI的检出率。另外, 全身MRI阳性的淋巴瘤BMI患者与全身MRI阴性的淋巴瘤BMI患者相比, 前者预后可能较差。     

Oxygen-induced inhibition of mouse brain lactate dehydrogenase.

The lactate dehydrogenase (LDH) activity of mouse brain homogenates was examined after exposure to hyperbaric oxygen (5763.8 mm Hg Po2) and compared to room air controls (158.8 mm Hg Po2). The effect of reduced glutathione on LDH activity after hyperbaric oxygen exposure was also examined. The activity of LDH after treatment with hyperbaric oxygen was significantly diminished when compared with controls. In the presence of reduced glutathione, homogenates exposed to hyperbaric oxygen demonstrated higher activity than did homogenates incubated without glutathione. It is concluded that oxygen-induced inhibition occurs through the oxidation of essential free sulfhydryl groups and that this oxidation can either be prevented by reduced glutathione or the disulfide bridges may be reduced to free sulfhydryl groups by the glutathione after oxidation.     

Marrow stromal cell recovery after radiation-induced aplasia in mice

PURPOSE: The identification of fibroblast-like cells of the marrow stroma by means of alkaline phosphatase (ALP) cytochemistry reveals delicate ALP-positive structures interspersed among haematopoietic cells and arranged in a loosely meshed network. These cells are often referred to as 'reticular' cells and the network they form is known as the 'ALP network'. The purpose was to analyse the evolution of this ALP network in relation to haemopoietic regeneration after whole-body irradiation. MATERIALS AND METHODS: The total surface occupied by ALP-positive processes revealed by means of ALP cytochemistry was expressed as a ratio of the total marrow area. ALP-positive cells were counted using nuclei as the defining unit. Cell proliferation was analysed by the detection of bromodeoxyuridine (BrdU) incorporation. Fat cells were identified by oil red O staining and alpha-glycerophosphate dehydrogenase (alpha-GPDH) activity. RESULTS: The ALP network and ALP-positive cell number began to increase 24 h after 4-Gy irradiation to reach a maximum after 72 h, when the bone marrow was almost completely empty of haemopoietic cells. This increase was in advance of haemopoietic recovery and was not due to cell proliferation. A decrease in the ALP network occurred in parallel with an increase in haemopoiesis and was accompanied by a transient increase in fat cells on day 7. CONCLUSIONS: These data indicate that the recovery of the ALP network, which is partially due to the recruitment of ALP- positive cells, occurs in advance of the haemopoietic recovery and that the equilibrium between fat cells and ALP-positive cells seems to be controlled by haemopoietic cells.     

多药耐药基因的人骨髓细胞转染及其对抗癌药的抗性增强作用

为探讨mdrl基因转染增强人骨髓细胞对抗癌药的抵御能力的可能性.通过脂质体介导,用含人mdrlcDNA表达质粒pHaMDR1/A,将mdrl基因转染人骨髓细胞,分别采用流式细胞术和免疫组化检测转染细胞mdrl基因的表达产物--P糖蛋白(Pgp),并用罗丹明试验证实其生物活性.同时,采用集落培养及抗性试验检测mdrl基因转染的人骨髓细胞对阿霉素、秋水仙碱、长春新碱和Vp16的抵御能力.结果mdrl基因被成功地导入了人骨髓细胞并获得了表达,罗丹明试验证实了Pgp具有完整的生物功能.mdrl基因转染的人骨髓细胞对上述抗癌药的抵御能力明显增强.     

The LD50 for uniform low LET irradiation of man

Previously published estimates of the whole-body radiation dose expected to kill 50% of a normal human population, the LD50, have rarely been based explicitly on evidence. The difference which might result from medical treatment seems to have been markedly over-valued. The available and relevant evidence about severe haematopoietic damage in man uncomplicated by tissue necrosis is indeed very scanty. It comes from 20 cases of therapeutic whole-body exposure to gamma rays and from two criticality accidents involving nine subjects, one of whom died, and when exposure was to neutrons as well as gamma rays. The observations suggest a judgment that 4.5 Gy (450 rad) absorbed dose in the bone marrow for energetic and therefore penetrating gamma rays giving reasonably uniform irradiation of the marrow could be regarded as the LD50 in circumstances where those irradiated were protected from thermal radiation and blast damage and from neutrons and beta rays. Examination of all the available experimental data on acute lethality following whole-body exposure to low LET radiation shows a remarkably similar co-efficient of variation of the LD50 in five species of large animal. If the same value is adopted for the human species, a quantitative estimate of the human LD50 can be inferred from the human evidence discussed. This reinforces the judgement that it is about 4.5 Gy. The use of observations after criticality accidents is examined in detail in an Appendix. The Vinca accident does not clearly meet the criteria for relevancy but there is no other instance of death in man from an uncomplicated brief whole-body exposure to ionising radiation where the bone-marrow dose can be estimated, and a possible judgment made of the value for the LD50.