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.