In this study, a monoclonal antibody (mAb) termed SN6 was generated by immunizing a mouse with a non-T-cell leukemia antigen preparation isolated from cell membranes of leukemia cells derived from a patient (FJ) with non-T/non-B-cell-type acute lymphoblastic leukemia (ALL). SN6 was tested against a variety of cultured and uncultured human cell specimens by using a sensitive cellular radioimmunoassay. Among the 26 cultured malignant and nonmalignant cell lines tested, SN6 reacted with all of the 6 leukemic non-T/non-B (including pre-B)-cell lines tested--i.e., KM-3, NALM-16, REH, NALL-1, NALM-1, and NALM-6. Of these cell lines, 5 were derived from individual patients with ALL; the remaining 1 was from a patient with chronic myelocytic leukemia in blast crisis. In addition, SN6 reacted with 3 of 3 leukemic myelo-monocytic cell lines tested--i.e., ML-2, HL-60, and U937. SN6 did not react with any other cell lines. A consistent result was obtained with 42 fresh (uncultured) cell specimens derived from individual patients with several different types of leukemias. SN6 reacted with 11 of 16 non-T/non-B (including pre-B)-cell ALL specimens. In addition, it reacted with various myelo-monocytic leukemia cell specimens to various degrees. SN6 did not show a significant reaction with normal peripheral blood cells tested, which included B cells, T cells, granulocytes, monocytes, and erythrocytes. However, it reacted with a small population (approximately 1% as determined by immunofluorescence staining) of normal bone marrow cells. The approximate molecular mass of the glycoprotein antigen defined by SN6 was determined to be 160,000 by radioimmunoprecipitation followed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Only one component of 80,000 daltons was formed upon reduction of the 160,000 molecular mass antigen. Therefore, this antigen is apparently a homodimer of a 80,000-dalton subunit. This conclusion was further corroborated by two-dimensional gel analysis, which showed a single well-defined spot for the reduced antigen. We designate this distinct human leukemia-associated cell surface antigen "GP160."