The diversity of function of mature circulating blood cells is reflected in their respective complements of cell-surface molecules and receptors. Although monoclonal antibodies have been instrumental in the identification and characterization of many cell-surface molecules on mature hematopoietic cells, the CD34 antigen represents to date, the only molecule, similarly identified, whose expression within the blood system is restricted to a small number of primitive progenitor cells in the bone marrow. Although its precise function remains unknown, the pattern of expression of the CD34 structure suggests that it plays an important role in early hematopoiesis. The availability of CD34 antibodies has greatly aided the development of techniques for the enrichment of primitive progenitor cells for studies of hematopoiesis in vitro. Additionally, the use of CD34 antibodies for the 'positive selection' of hematopoietic stem/progenitor cells represents and alternative strategy to 'negative selection' or purging for the large-scale manipulation of bone marrow cells prior to transplantation. The availability of pure populations of the most primitive hematopoietic progenitor cells may also facilitate the development of genetic techniques for the repair of specific blood cell disorders. In this article, we review the biology of the CD34 molecule and assess some of the roles for CD34 antibodies in immunopathology and for progenitor/stem cell purification in clinical applications.