A monoclonal antibody (MoAb) recognizing a membrane-associated erythroid burst-promoting factor was prepared by immunizing BALB/c mice with plasma membrane-derived vesicles exfoliated from lymphocytes under serum-free conditions. Hybrids secreting antibody reactive with lymphocyte plasma membranes were formally cloned and IgG was purified from monoclonal supernatants or from BALB/c mouse ascites fluid. Two clones (D3-E4 and D3-G9) were found to suppress burst forming unit- erythroid (BFU-E) proliferation when added directly to serum-free human marrow culture. Inhibition to a level of 100% was observed in a dose- dependent fashion over a wide range of antibody concentrations (0–200 micrograms/mL). Neither antibody altered the proliferation of colony forming unit-granulocyte macrophage (CFU-GM) or colony forming unit- granulocyte-erythroid-monocyte-megakaryocyte (CFU-GEMM) progenitor cells in human bone marrow culture. The D3-E4 clone was found to produce an IgG1 antibody which adsorbs an erythroid burst-promoting activity (BPA) from supernatants of, and octylglucoside extracts of shed vesicles present in, serum-free, lymphocyte conditioned medium (LCM), and which recognizes a vesicular protein of Mr approximate 30,000 on immunoblots of membrane proteins electrophoresed on sodium dodecyl sulfate (SDS)/polyacrylamide and transferred to nitrocellulose. In contrast, the D3-G9 clone was found to produce an IgG1 cytotoxic antibody. These antibodies will be important to the study of cell-cell and growth factor-cell interactions in vitro.