We studied the expression of 2H4 and 4B4 on the surfaces of leukemia cells from 17 patients with adult T-cell leukemia (ATL) as well as of cells belonging to 2 T-cell lines derived from ATL patients. The effects of the supernatants obtained from culture fluids of the ATL cells and the T-cell lines on IgG production of a human B-cell line, CESS cells, were also examined. On the surfaces of the ATL cells from 15 out of 17 cases and of the cells of 2 T-cell lines 4B4 obviously existed at higher percentage than 2H4 and more than 80% of ATL cells from 16 out of these 17 cases showed the expression of T4 (CD4). These findings revealed that the most of ATL cells had a helper-inducer phenotype. Supernatants (Sups) of culture fluids of ATL cells from 4 patients and those of 2 T-cell lines were added at various concentrations to the CESS cells. In only 1 Sup from ATL patient enhanced the IgG production of the CESS cells at lower concentration. However, other 5 Sups suppressed the IgG production of the CESS cells in proportion to the increase of Sup added. These results showed that phenotypical type of ATL cells does not always correspond to their functions, and the ATL cells may produce humoral factors that regulate B cell functions.