An Inhibitor of Stem Cell Proliferation in Normal Bone Marrow

S ummary . A saline extract from normal bone marrow cells having a molecular weight in the range 50 000–100 000 daltons has been found to protect rapidly proliferating haemopoietic spleen colony forming cells (CFU_s) from the lethal effects of large doses of tritiated thymidine. This extract is non-toxic to the cells. It is not found in regenerating marrow where the CFU_s population is rapidly proliferating. Its effect appears to be specific for the CFU_s since it has no effect on the proliferation of its close descendant, the granulocytic precursor cell (CFU_c), and no effect on the average cytoplasmic structuredness of the whole bone marrow cell population. The active material is probably protein since it is degraded by trypsin.