Despite the wide variety of functions exhibited by mature peripheral blood cells, all are derived from a small pool (1-3%) of primitive precursor cells in the bone marrow (BM) that bear a unique surface glycoprotein, CD34. Isolated CD34+ cells are capable of reconstituting all hematopoietic lineages, both in experimental animals and in humans following intensive therapy. CD34+ cells capable of reconstituting hematopoiesis are also found at low frequency in peripheral blood (PB), a frequency which can be dramatically increased by combinations of chemotherapy and recombinant cytokines. In some cases, PB "stem cells" (PBSC) can be used to augment or even replace conventional BM autografts. The availability of CD34 antibodies has greatly aided the development of techniques for the enrichment of primitive progenitor cells, thus allowing studies of the hematopoietic potential of stem cells in vitro. Additionally, the use of CD34 antibodies for the "positive selection" of hematopoietic stem/progenitor cells from tumor-contaminated marrow may possibly represent an alternative "purging" strategy prior to transplantation. The availability of pure populations of the most primitive hematopoietic progenitor cells will also facilitate study of genetic manipulation as a practical therapeutic modality.