Surface phenotypes of leukemic cells from 16 patients with adult T-cell leukemia/lymphoma (ATLL) were analyzed by using monoclonal antibodies (anti-Leu-1, anti-Leu-2a, anti-Leu-3a, anti-HLA-DR and MAS 036 c), and the effect of leukemic cells on PWM-induced normal B-cell differentiation was also studied. The majority of ATLL cells bear Leu-1 and Leu-3a antigen on cell surface but lack Leu-2a antigen and were unreactive with MAS 036 c. These results indicate that ATLL cells are of peripheral inducer/helper T-cell origin. However, contrary to the surface phenotype, ATLL cells from 10 patients showed potent suppressor activity on PWM-induced normal B-cell differentiation to immunoglobulin-producing cells (Ig-PC) and no case showed helper activity. The dissociation between surface phenotype and function of ATLL cells is discussed in this article.