Complete hematopoietic reconstitution using nonleukemic peripheral blood mononuclear cells has yet to be achieved in humans. Significant advances have been made in in vitro quantification of putative stem cells, in animal models of the reconstitutive process, and in the collection, processing, and storage techniques for stem cell preparations. These may eventually lead to use of hematopoietic stem cells from peripheral blood for both allogeneic and autologous bone marrow reconstitution in a variety of clinical situations.