Two new cell surface antigens specific for the erythroid lineage were defined with cytotoxic IgM monoclonal antibodies (McAb) (EP-1; EP-2) that were produced using BFU-E-derived colonies as immunogens. These two antigens are expressed on in vivo and in vitro derived adult and fetal erythroblasts, but not on erythrocytes. They are not detectable on resting lymphocytes, concanavalin-A (Con-A) activated lymphoblasts, granulocytes, and monocytes or granulocytic cells or macrophages present in peripheral blood or harvested from CFU-GM cultures. Cell line and tissue distributions distinguish McAb EP-1 and EP-2 from all previously described monoclonal antibodies. McAb EP-1 (for erythropoietic antigen-1) inhibits the formation of BFU-E and CFU-E, but not CFU-GM, colonies in complement-dependent cytotoxicity assays. By cell sorting analysis, about 90% of erythroid progenitors (CFU-E, BFU-E) were recovered in the antigen-positive fraction. Seven percent of the cells in this fraction were progenitors (versus 0.1% in the negative fraction). The expression of EP-1 antigen is greatly enhanced in K562 cells, using inducers of hemoglobin synthesis. McAb EP-2 fails to inhibit BFU-E and CFU-E colony formation in complement-dependent cytotoxicity assays. EP-2 antigen is predominantly expressed on in vitro derived immature erythroblasts, and it is weakly expressed on mature erythroblasts. The findings with McAb EP-1 provide evidence that erythroid progenitors (BFU-E and CFU-E) express determinants that fail to be expressed on other progenitor cells and hence appear to be unique to the erythroid lineage. McAb EP-1 and EP-2 are potentially useful for studies of erythroid differentiation and progenitor cell isolation.