PSK is a protein-bound polysaccharide prepared from cultured mycelium of Coriolus versicolor. The effects of PSK on immunologic responsiveness were investigated in C3H/He mice bearing syngeneic X5563 tumor. The results were as follows. elayed foot pad reaction and antibody-forming capacity to sheep erythrocytes were depressed in tumor bearing mice, and such depression was prevented by oral or intraperitoneal administration of PSK. In vitro cytotoxic activity of splenic lymphocytes against the tumor was augmented by PSK administration. Antitumor effect was augmented by combination of PSK and X-irradiation. Delayed foot pad reaction to sheep erythrocytes was suppressed in normal C3H/He mice given immunosuppressive substance obtained from tumor-bearing mice, and the depressed reaction recovered to the normal level following PSK administration. These results show that PSK is effective in the syngeneic murine tumor system.