The ability to isolate large numbers of hematopoietic progenitors will facilitate an understanding of the growth and differentiation of bone marrow. Furthermore, isolating hematopoietic progenitors will have widespread clinical applications to autologous marrow transplantation, allogeneic marrow transplantation, gene therapy, and in vitro marrow expansion. With the development of avidin-biotin immunoadsorption, it is now feasible to isolate large numbers of these progenitor cells for clinical purposes. Successful hematopoietic reconstitution has been demonstrated in lethally irradiated baboons transplanted with CD34+ cells isolated by immunoadsorption with the anti-CD34 antibody 12-8. Recent studies have shown that CD34+ cells enriched from the marrow of patients with metastatic breast cancer can be used for autologous marrow transplantation.