A culture system was used to analyze the expression of membrane differentiation antigens on the proliferative or clonogenic fraction of cells from cases of common (precursor B) acute lymphoblastic leukemia (c-ALL). Colonies of leukemic cells were obtained in 18 of 20 cases after 1 week in culture in a liquid layer containing recombinant interleukin-2 (IL-2), phytohemagglutinin (PHA), and B-cell growth factor over an agar feeder layer containing irradiated peripheral blood mononuclear cells plus fetal calf serum (FCS) and horse serum. Cultured cells expressed HLA-DR, CD-9, CD-10, CD-20, and CD-34 antigens, indicating conservation of precursor B phenotype. Differentiation antigen expression on the clonogenic subpopulation giving rise to leukemic colonies was assessed by treating cells prior to culture with selected cytotoxic monoclonal antibodies (MoAbs) and complement or by fluorescence-activated cell sorting. Lytic treatment with HLA-DR, CD-10, CD-9, and CD-20 antibodies produced median reductions in colony formation of 93%, 81%, 73%, and 58% respectively. Combined treatment with CD-9 and CD-10 antibodies produced complete inhibition in 11 of 13 cases. Cell-sorting experiments after CD-10 staining indicated a good correlation with complement lysis studies and also demonstrated that the CD-19 antigen is expressed on a high proportion of clonogenic cells. Colony formation was also significantly reduced in 13 of 17 cases by lytic treatment of cells with the CD-33 antibody MY-9, suggesting expression of this "myeloid" lineage antigen on ALL clonogenic cells. The substantial case-to-case variation in expression of individual differentiation antigens indicates that leukemic cellular heterogeneity is a major consideration in ex vivo bone marrow purging using MoAbs and emphasizes the need for more critical evaluation of purging techniques.