RFB-1 is a monoclonal antibody previously shown to react with granulocyte-monocyte progenitors (CFU-GM) and immature lymphoid cells in human bone marrow. RFB-HLA-DR is a monoclonal antibody that reacts with HLA-DR (la-like) antigens. The present study shows that the bone marrow subset reactive with both RFB-1 and RFB-HLA-DR contains all the cells that give rise to mixed hematopoietic colonies (derived from CFU-GEMM; a pluripotent human progenitor cell) as well as to megakaryocytic (megakaryocyte-CFU-derived) and erythropoietic (derived from erythroid burst-forming units, BFU-E) colonies, as shown by fluorescence-activated cell sorting and complement-mediated cytotoxicity. These results indicate that CFU-GEMM, BFU-E, and megakaryocyte-CFU express RFB-1 and la-like antigens. RFB-1 antigen is also expressed on erythroid colony-forming units (CFU-E). RFB-1 and RFB-HLA-DR are useful reagents in the study of hematopoietic stem cells.