The previously reported decrease in the ratio of erythrocytic (E): granulocytic (G) spleen colonies observed 8 days after bone marrow transplantation, when the donors had been x-irradiated up to 4 hours before the marrow was taken for transfer, was confirmed. The effect of taking the marrow between 4 hours and 13 days after x-irradiation, on the E:G ratio in both spleen and femoral bone marrow colonies, has been studied in detail. The reversion to a normal E:G ratio in spleen colonies assessed at 8 or 10 days was associated with increases in this ratio of colonies in the femoral bone marrow. Transplantation of cell suspensions prepared from the spleen and femoral bone marrow of primary recipients 8 days after the initial transfer produced a histological distribution of 8-day colonies in secondary recipients which was indpendent of x-irradiation of the donor. The data suggest that a hypothesis involving CFU migration within the spleen and, possibly, from the bone marrow to the spleen, provides a more satisfactory explanation of the data than one involving two CFU populations with different radiosensitivities.