Six murine monoclonal antibodies reactive with human myeloid lineage differentiation antigens are described. These antibodies, designated WM-12, WM-14, WM-15, WM-16, WM-19, and WM-20, react with normal peripheral blood neutrophils and monocytes, as well as a proportion of myeloid precursor cells in bone marrow, but fail to react with the majority of normal lymphoid cells. Immunoprecipitation studies have demonstrated binding of WM-12, -14, -16, -19, and -20, to elements of a protein multimer with sub-units of 50, 105, and 170 kilodalton molecular weight under reducing conditions. WM-15 antibody, however, reacts with a separate protein of 165 kilodaltons. These 6 antibodies reacted with 89% of cases of acute myeloid leukemia, but showed significant binding with only occasional cases of acute lymphoblastic leukemia. Cases of AML (FAB M1 and M2) were more frequently positive with WM-15 (60% of cases positive) than with the other 5 antibodies, whereas the converse was true for leukemias with monocytic differentiation (FAB M4 and M5; 82-100% of cases positive with WM-12, -14, -16, -19, -20). These antibodies appear useful for diagnosis and classification of acute leukemia.