Abstract
A saline extract from normal bone marrow cells having a molecular weight in the range 50000-1000000 daltons has been found to protect rapidly proliferating haemopoietic spleen colony forming cells (CFUs) 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 CFUs population is rapidly proliferating. Its effect appears to be specific for the CFUs since it has no effect on the proliferation of its close descendant, the granulocytic precursor cell (CFUc), 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.
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