Transplantable human hematopoietic stem cells (competitive repopulating units [CRU]) can be quantitated based on their ability to produce large populations of lymphoid and myeloid progeny within 6 weeks in the marrow of intravenously injected, sublethally irradiated NOD/SCID mice. It is shown that the proportions of total injected human fetal liver and cord blood CRU in the marrow of mice 24 hours after transplantation are 5% and 7%, respectively, as determined by limiting-dilution assays in other primary and secondary NOD/SCID mice. The similarity in these 2 seeding efficiency values suggests that mechanisms regulating the ability of human hematopoietic stem cells to enter the marrow from the blood, at least in this xenotransplant model, do not change between fetal life and birth. In addition, it appears that previously reported human stem cell frequencies and their in vivo self-renewal activity measured in NOD/SCID mice have been markedly underestimated.