The authors have developed a murine monoclonal antibody, RC-82.4, against an antigen expressed by a human erythroleukemia cell line OCI-MIR. The antibody reacts with an antigen expressed by proerythroblasts, normoblasts, and some reticulocytes but not expressed in erythrocytes, granulocytes, monocytes, megakaryocytes, plasma cells, or lymphocytes. The authors have established an immunocytochemical method for studying bone marrow smears with RC-82.4. By studying bone marrow smears from 11 patients with M-6 erythroleukemia and 104 patients with various other hematologic and nonhematologic malignancies, the authors have found that RC-82.4 has great sensitivity and specificity in recognizing erythroid differentiation in blasts. The authors have used RC-82.4 and antihemoglobin antibodies to identify erythroblasts in acute and secondary acquired cases of erythroleukemia that would have been unclassifiable by morphologic and all other conventional cytochemical and immunocytochemical criteria.