Data pertaining to the endogenous mouse spleen colony system, 10 days postirradiation, are presented. The Do for visible colonies is 78 R, while that for 6 hr iron uptake over a range of 600–800 R is 50 R. The Do for spleen weight is 196 R and that for IUdR uptake is 193 R. These measurements increase with the age of the mouse. Hypertransfusion decreases spleen iron uptake and colony number. DF-32P and sodium sulfate-35S are not useful indicators of splenic hematopoiesis in this system. Visible hematopoietic colonies in the spleen are not produced by vinblastine, nitrogen mustard, methotrexate, or cyclophosphamide.

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