Human umbilical cord blood (UCB) may be used as an alternative source of bone marrow repopulating cells in allogeneic bone marrow transplantation in children. It has been reported that high numbers of hematopoietic progenitor cells (HPC) from umbilical cord blood may be lost during simple cell-separation techniques. This may seriously hamper the use of UCB as an alternative source of bone marrow repopulating cells. In this study we demonstrate that UCB can be separated into various cell fractions using several cell-separation methods including red blood cell lysis, methylcellulose sedimentation, and density gradients without significant loss of HPC, when cell separations are initiated within 8 h. We demonstrate that UCB contains a high concentration of immature HPC as compared with bone marrow grafts. Using FACS analysis of cells harvested from single colonies derived from single cell- single well-sorted CD34++ CD33- UCB cells, the high frequency of multipotential HPC was illustrated. These results suggest that UCB may contain sufficient HPC for hematopoietic stem cell transplantation in adults.