A monoclonal antibody (McAb) designated 3A2 that recognizes a 51 kDa epitope having surface density of 37 x 10(8) per MOLT-4 cells is described. This epitope appears to be expressed on (i) lymphocytes at all stages of differentiation; (ii) leukemic myeloid progenitors; (iii) peripheral blood monocytes (MO). The epitope is specifically absent from normal myeloid progenitors and macrophages. The McAb may, therefore, be useful in studying myeloid lineage leukemias and, as a marker for monocyte to macrophage (MO + MAC) differentiation.