The effects of PSK, a protein-bound polysaccharide, on the survival period and effector cell activity were examined using C57BL/6 mice with melanoma B16. PSK prolonged the survival period of the mice with tumors in a schedule- and dose-dependent manner. However, no life-prolonging effect was observed when carrageenan-treated mice or congenitally athymic mice were used as hosts. PSK enhanced the cytostatic activity and interleukin-1-producing capacity of peritoneal exudate plastic-adherent cells in C57BL/6 mice with tumors. These findings suggested that PSK prolongs the survival period of mice with B16 tumors through T-cell- and macrophage-dependent mechanisms.