The cellular mechanism of immunosuppression induced by 7,12-dimethylbenz[a]anthracene (DMBA) was studied by using both in vivo and in vitro assay systems. When the effect of DMBA on antibody responses was studied by the use of an adoptive cell transfer system of primed cells with second antigen in X-irradiated recipient mice, the development of both hapten-primed B cells and carrier-primed helper T cells induced by thymus dependent hapten-carrier conjugate was found to be markedly suppressed in DMBA-treated mice. When the B cell activity in DMBA-treated mice was directly assessed using thymus-independent hapten-carrier conjugate, anti-hapten antibody response was not suppressed at all. Since the development of hapten-primed B cells to thymus-dependent carrier requires the presence of carrier-primed helper T cells, DMBA treatment seems to affect primarily T cells but not B cells, and the suppression of development of hapten-primed B cells may be a secondary phenomenon induced by the suppressed helper T cell activities. Since the development of helper T cell activities requires antigen presentation by macrophages, it was determined which type of T cells or macrophages was the primary target of DMBA-induced immunosuppression. Antigen-primed T cells from DMBA-treated mice did not proliferate as well on stimulation with secondary antigen in vitro. Furthermore, antigen-pulsed macrophages from DMBA-treated mice did not fully activate T cells from normal mice. Thus, the primary target of DMBA-induced immunosuppression was considered to be on antigen-presenting macrophages rather than on T cells.