Antibody-secreting hybridomas were produced by fusion of P3-NS1/1Ag--4-1 mouse plasmacytoma cells with splenocytes from a mouse immunized with HL-60 human promyelocytic leukemia cells. A cloned hybridoma cell line was identified that secreted antibody against a cell surface antigen expressed on all normal human peripheral blood granulocytes, on bone marrow granulocytic precursor cells, and on blast cells from 3 of 15 patients with nonlymphoid leukemia. However, the antibody did not react with normal peripheral blood lymphocytes, monocytes, platelets, or red cells, or with blast cells from 20 patients with acute lymphoblastic leukemia or from 5 patients with lymphoma. This monoclonal antibody identified a granulopoietic differentiation antigen (designated My-1) and may prove useful in the subclassification on nonlymphoid leukemia and in the investigation of hematopoiesis.