Kinetics of competitive blood repopulation after transplantation of SP cells purified from adult bone marrow, liver, or peripheral blood. Purified cell populations from CD45.1 mice were transplanted in competition with 2 × 10⁵ unfractionated bone marrow cells into lethally irradiated CD45.2 mice. Average peripheral blood chimerism arising from the donor test cells was followed monthly. Two hundred liver SP (LiSP) tip CD45⁺ cells demonstrated potent hematopoietic-repopulation activity that approached bone marrow (BM) SP (P = .08). In contrast, nucleated peripheral blood cells, blood SP cells, and LiSP shoulder CD45⁺ cells possess limited, if any, competitive repopulation potential compared with bone marrow SP (P < .05) (n = 4 per group per experiment repeated twice; average chimerism at 3 months is enumerated above each bar; bars represent mean chimerism and error bars show SEM).