We investigated the expression of CD4 antigen in normal bone marrow (BM) samples, enriched in CD34+ hematopoietic progenitor cells. At flow cytometry, a significant fraction (ranging from 25% to 65%) of CD34+ cells also showed low levels of CD4 antigen on their surface. The CD4 receptor densities on the surface of hematopoietic progenitors was approximately 50% that of peripheral blood monocytes and 5% of peripheral blood T lymphocytes. In immunoprecipitation experiments, the CD4 antigen expressed by BM hematopoietic progenitors appeared to be the same form expressed by mature peripheral blood CD4+ cells and appeared to be a potentially functional receptor for human immunodeficiency virus-type 1, because it specifically bound recombinant envelope gp120. Moreover, BM samples, highly enriched in CD34+ cells, showed the presence of CD4 mRNA at reverse transcription- polymerase chain reaction examination. In experiments of complement- mediated cytotoxicity with Leu3-a+Leu3b anti-CD4 monoclonal antibody, a significant reduction in the number of both classes of megakaryocyte (burst-forming unit-meg [BFU-meg] and colony-forming unit-meg [CFU- meg]) and granulocyte/macrophage (CFU-GM) progenitors was observed, whereas erythroid (BFU-E) progenitors were only slightly affected. Moreover, purified CD4+ BM cells obtained by immunomagnetic selection, using high concentrations of Leu3a+Leu3b, showed a colony-forming ability of megakaryocyte and granulocyte/macrophage progenitors comparable with that of CD4- BM cells. In conclusion, the present data show that immature hematopoietic progenitor cells express low levels of CD4, the high-affinity receptor of human immunodeficiency virus-type 1.