The cellular site of immunosuppression in Ehrlich tumor-bearing mice was analysed with particular reference to the T- and B-cell activities. The B-cell activity as measured by the anti-dinitrophenyl (DNP) antibody responses to DNP-thymus-independent carriers (TID) was not impaired in tumor-bearing mice as compared with normal mice, whereas the anti-DNP antibody response to DNP-thymus-dependent carriers (TD) and the development of helper T-cell activity to TD were markedly suppressed in tumor-bearing animals or mice pretreated with cell-free cancerous ascitic fluid. The selective suppression of T-cell response was not mediated by the generation of suppressor cell activity toward TD, which may depress the manifestation of developed helper T-cell activity. A marked suppression of T-cell response was observed when the animals were inoculated with tumor cells or injected with cancerous ascitic fluid prior to antigenic stimulation, but not when the animals were rendered tumor-bearing by such treatments after the immunization. The suppression of T-cell activity in both sarcoma 180 tumor-bearing mice and cell-free Ehrlich cancerous ascitic fluid-treated mice was prevented by treatment with lentinan, a potent anti-tumor polysaccharide. The applicability of this experimental system to the search for immunopotentiators relevant to tumor immunotherapy is discussed in the light of the preventive effect of lentinan on the suppression of T-cell response in tumor-bearing animals.