A newly developed monoclonal antibody (Y1/82A), which binds a cytoplasmic antigen in peripheral blood monocytes and tissue macrophages, was tested for its ability to detect monocytic differentiation in acute myeloid leukemia, i.e., to distinguish French-American-British (FAB) groups M4 and M5 from FAB groups M1, M2, M3, M6, and M7. Staining was performed by the alkaline phosphatase-antialkaline phosphatase (APAAP) immunocytochemical technic on bone marrow smears from 29 cases of acute myeloid leukemia, on 17 normal peripheral blood and/or bone marrow smears, on bone marrows from 10 cases of lymphoid leukemia, and on lymph nodes of 13 patients with lymphoma. Neoplastic cells from 11 of 11 patients with either M4 or M5 leukemia had positive results, whereas only 2 out of 18 cases of M1, M2, M3, M6, and M7 leukemia had positive results. In normal samples, only peripheral blood monocytes, bone marrow macrophages, and megakaryocytes stained. Lymphoid neoplasms were unreactive. These results suggest that monoclonal antibody Y1/82A may be a useful reagent in detecting cases of M4 and M5 acute myeloid leukemia and that it offers a valuable alternative to nonspecific esterase cytochemistry.