Homing of murine BM cells. 10⁸ PKH26-labeled BM cells were injected into lethally irradiated hosts. At the indicated times, recipient BM and spleen cells were counted to determine whole-organ cellularity, and the frequency of PKH26⁺cells in each determined by FACS. Labeled cells were sorted and assayed for CFCs together with an aliquot of labeled cells from the pretransplant suspension. Shown are the mean ± SEM percent recoveries of total cells (A) and CFCs (B) in BM (○) and spleen (•) for pooled data from 3 experiments (3 mice/time point). Values are derived from the formulae: % Cell Recovery = (% PKH26⁺ by FACS × whole-organ cellularity)/10⁸, and % CFC Recovery = (CFC frequency in sorted PKH26⁺ cells × No. PKH26⁺ cells per organ)/(CFC frequency in pretransplant BM × 10⁸). Differences in the recovery of total cells and CFCs is significant at 24 hours (P < .05); differences in the recovery of CFCs at 3 hours is significant to P = .1.