A monoclonal antibody, 12-8, prepared against KG-1a cells, recognizes an approximately 115-kd cell surface antigen and reacts with 3% to 4% of bone marrow cells, including most of the blast cells. The antigen is not expressed on peripheral blood cells. Marrow cells expressing 12-8 collected by fluorescence-activated cell sorting contained nearly all of the unipotent (CFU-GM, BFU-E) and multipotent (CFU-MIX) colony-forming cells. The isolated 12-8 positive marrow population also contained precursors of these colony-forming cells. In a two-stage long-term marrow culture system employing irradiated allogeneic marrow adherent cells, 12-8 positive cells produced both unipotent and multipotent colony-forming cells for ten weeks. Moreover, the output of colony forming cells substantially exceeded the input. Antibody 12-8 appears useful for analysis and possibly enrichment of hematopoietic progenitor cells that include colony-forming cell precursors.