The proliferation rate of murine hematopoietic cells in regenerating spleen was assayed by a novel technique that analyzes the integrity of stem cells irrespective of their number. It relies on the increment of incorporation of 5-(125I) iodo-2'-deoxyuridine (125IUdR) at day 3 and 5 after transfusion of syngeneic marrow cells in fatally irradiated recipients. A prerequisite of the assay is the linearity between in fatally irradiated recipients. A prerequisite of the assay in the linearity between 125IUdR incorporation in the spleen and the number of cells transfused at both days of observation. The average increment of activity of 125IUdR from day 3 to 5 for the various graft size permits the description of a proliferation factor and from that a doubling time of the proliferating population in the spleen. Whole-body gamma-irradiation of donor mice causes a significant increase of doubling time in the grafts which persisted in part for 7 months after recovery from 500 rad exposure. The prolongation of doubling time expresses a residual injury and may interfere with the regulation of proliferation.