The repopulating ability and homing potential of CD34+ cells isolated from either bone marrow (BM) or umbilical cord blood (UCB) was compared in NOD/SCID mice. Mice were sublethally irradiated (3.5 Gy) and within 24 h transplanted iv with 10 (5)-10 (6) CD34+ cells. Four weeks post-transplantation blood was collected from the tail vein for detection of human cells and after 6-7 weeks the mice were sacrificed and blood, BM, thymus, lymph nodes, spleen, liver, lung and kidney were harvested and single cells suspensions were made. Human cells were detected by flow cytometry, and staining was performed for CD34, CD45, and markers of the myeloid, and lymphoid lineages. CD34+ cells from UCB successfully engrafted into the NOD/SCID mice. Eighty-five percent of cells in BM of the mice were of human origin, depending on the dose of cells injected. In all other organs these percentages were always lower, and maximally 63, 13, 3 and 2% in spleen, liver, lung and kidney, respectively. Transplantation of CD34+ cells isolated from human BM, on the other hand, resulted in a very low percentage of human cells after 6-7 weeks of transplantation, not exceeding 3% of the cell suspension. Whether this difference in repopulating ability can be explained by an intrinsic qualitative difference or by differences in quantity of stem cells within the CD34+ compartment from UCB or BM remains to be determined.