The immunosuppressive activity of sera from adult T cell leukemia (ATL) patients and culture supernatants of ATL cells and ATL cell lines on B cell responses was examined in vitro. ATL sera and culture supernatants of ATL cells suppressed B cell proliferative and differentiative responses induced with B cell mitogens such as Staphylococcus aureus Cowan I and pokeweed mitogen. The suppressive effect was observed not only in the production of B cell growth factors (BCGF) and B cell differentiation factors (BCDF) by T cells, but also in the responsiveness of B cells to BCGF and BCDF. These results suggest that the immunosuppressive factors produced by ATL cells might act to suppress both the cellular immunity and the humoral immunity, and could have an important role in the induction of immunodeficient states in ATL patients.