[en] In order to detect injury to stem cells which neither kills them nor inhibits their reproductive integrity, an assay method was used which measures the proliferative ability of irradiated bone marrow relative to unirradiated bone marrow, following transfusion into lethally irradiated recipient mice. Neither the proliferation factor nor the CFU-S number per femur recovered completely within one year after 5 Gy, and the number of CFU-S per femur was more suppressed than the PF. (UK)