Classically hemopoietic stem cells to be used for transplantation or autologous reinfusion have been harvested from the bone marrow which has remained the major source of stem cells for allogeneic transplantation. However, pluripotent stem cells also circulate in peripheral blood under physiological conditions and can be "mobilized" to appear in very large numbers in peripheral blood by treatment with cytotoxic chemotherapy, hemopoietic growth factors, or both. These cells are now being used widely for autologous stem cell support. They have the advantage of very rapid hemopoietic reconstitution, thereby shortening the posttransplant period of pancytopenia. Fetal liver cells, another classic source of stem cells, are currently used only infrequently. However, there is a growing interest in the use of umbilical cord blood which is rich in stem cells and easily accessible. Cord blood stem cells have been used successfully for pediatric transplants even across major histocompatibility barriers. Technology has been developed which may permit sufficient in vitro expansion so that these cells can also be used for transplants in adults. Furthermore, there is evidence that these cells may be preferable to marrow or even mobilised peripheral blood stem cells for the purpose of gene transfer.