This study was carried out to determine the reactivity of the Leu-1 mouse monoclonal antibody with B-chronic lymphocytic leukemia and other B-cell leukemias. This antibody has been previously reported to recognize a surface antigen expressed by almost all human thymocytes and peripheral T cells. It was also detected on surface immunoglobulin-bearing cells of most patients with chronic lymphocytic leukemia but was not detectable in normal B-cells and B-cell lines. In the present series, the neoplastic lymphocytes in 33 cases of B-chronic lymphocytic leukemia expressed surface immunoglobulin, Ia antigen, and receptors for Fc, C3, and mouse erythrocytes. All but one case expressed the Leu-1 antigen as detected by indirect immunofluorescence using flow cytometry. In three other cases, the leukemic cells in the peripheral blood reacted with anti-Leu-1, whereas the bone marrow lymphocytes did not. Moreover, quantitative differences in the surface density of Leu-1 were apparent by flow cytometry. The peripheral blood and bone marrow lymphocytes in other B-cell leukemias, including 15 cases of leukemic B-cell lymphomas and three cases of hairy-cell leukemia, failed to stain positively with the anti-Leu-1 antibody. The recognition of the Leu-1 antigen adds to the phenotypic characterization of b-chronic lymphocytic leukemia and may contribute to a better understanding of the pathogenesis of the disease and its expression in different tissues.