Experimental design for competitive repopulation studies. Lethally irradiated young or old Ly-5.1 mice were intravenously injected with 3 × 10⁶ old or young Ly-5.2 BM cells. Twenty-four hours later, marrow from the primary recipients was harvested to assay the number of Ly-5.2⁺ stem cells that had homed there after transplantation. The frequencies of CRUs in the initial BM suspensions and in the BM of primary mice after homing were determined by competitive repopulation of 2 sets of Ly-5.1 mice that underwent transplantation with graded numbers of fresh or homed BM cells admixed with 2 × 10⁵ Ly-5.1 competitor BM cells. Donor (Ly-5.2)-derived lymphoid and myeloid cells in PB were then measured at 5, 10, 17, and 26 weeks after transplantation. CRU frequencies were calculated from the proportions of negative mice in each cell-dose group, as described in “Materials and methods.”