Cell surface antigens of the human bone marrow CFU-C have been studied. Human marrow cells were incubated with a variety of monoclonal antisera and complement prior to culture in semisolid media. By using indirect immunofluorescent studies, the percentage of bone marrow cells binding the antibodies was determined. The CFU-C phenotype is HLA+, la+, 4F2+, 3A1-, and DUALL-1-. This study provides information that is useful in the study of myeloid cell ontogeny and necessary for the use of some of these reagents in the treatment of bone marrow cells prior to human bone marrow transplantation in various clinical settings.