The seeding of transplanted CFUc and CFUs in the spleen and femur of irradiated mice has been investigated in order to compare their kinetic behavior following transplantation. The conditions in which the seeding of transplanted CFUc into lethally irradiated recipient mice can be studied were determined. Irradiated femoral or splenic cells produce potentiation of colony growth when added in relatively low numbers and result in serious inhibition at higher concentrations. By using a preirradiated recipient in which the postirradiation cell loss has been completed, rather than one that has only just been irradiated, changes in plating efficiency during the course of the experiment are prevented in the spleen and are maintained constant in the femur. It is shown that significant numbers of CFUc and CFUs quickly appear in both the spleen and bone marrow after transplantation. A fall in CFUc numbers occurs after 2-3 hr both in femur and spleen, while CFUs numbers remain stable. The possible mechanisms involved are considered, and differentiation is suggested as the cause of the fall.