Objective. To determine the bone marrow homing efficiency (20 hours) of cultured compared to noncultured umbilical cord blood (UCB)-derived human hematopoietic cells in the nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse, and to explain the difference in homing between these populations. Methods. Human UCB CD34+ cells were cultured for up to 5 days, reselected, and used for transplantation, phenotype analysis, and functional studies, including adhesion and trans-endothelial migration assays. Seeding of CD34+ cells was measured after labeling of cells with 111-Indium, while homing of colony-forming cells (CFC) and SCID-repopulating (SRC) cells was determined using functional assays. Results. Short-term culture was associated with a decrease in the 20-hour homing of CD34+ cells, CFC, and SRC to the BM. Although cultured compared to noncultured cells showed increased expression and function (adhesion/migration) of several cell adhesion molecules described to play a role in homing and engraftment, culture also induced expression of Fas/CD95 and rendered cells more susceptible to apoptosis. Finally, we demonstrate that the level of Fas/CD95 on cultured cells was inversely related to the ability of CFC to home to the BM, and that the homing of cultured CFC could be restored by incubating cells prior to transplantation with Fas/CD95-blocking mAb ZB4. Conclusion. These data implicate Fas/CD95 in the homing defect of cultured human hematopoietic cells in the NOD/SCID transplant model and suggest that prevention of apoptosis may be an important strategy to improve engraftment of ex vivo-manipulated HSC in a clinical setting.