Frequency and recovery of lineage-negative cells among Sca-1⁺ donor cells in BM, spleen, and PB for times up to 20 hours following transplantation of low-density BM cells in irradiated and nonirradiated recipients. Mice underwent transplantation with 1 × 10⁷ to 1 × 10⁸ LDBM cells, were killed at the indicated time points, and were analyzed for frequency and recovery of donor Sca-1⁺ lin^– cells in BM, spleen, and PB as described in “Materials and methods.” Data in panels A and C are expressed as the mean ± SEM percent of donor Sca-1⁺ cells falling within a light scatter gate including lymphocytes and large granular cells and that lack lineage expression. Horizontal lines in panels A and C represent the mean percentage of lineage-negative donor Sca-1⁺ cells in the original graft. Using a similar light scatter gate, recoveries of Sca-1⁺ lin^– donor cells were calculated and are shown in panels B and D. Bars in panels B and D represent mean ± SEM recovery of Sca-1⁺ lin^– donor cells (sum of recoveries in BM plus spleen plus PB), and lines represent mean ± SEM recoveries in BM, spleen, or PB. Numbers above each bar represent mean recoveries. n = 6-12 in panel A; n = 4-11 in panel B; n = 4-5 in panel C; n = 3-5 in panel D. *P < .05 when compared with spleen of the same time point. To provide an SEM value for total recovery at each time point, total recoveries were not calculated by adding the means of the recoveries from each tissue but rather by averaging the total recoveries from each individual experiment.