Substantial degrees of resistance against rechallenge with viable tumor cells and cytotoxic activity of peritoneal exudate cells (PE cells) as detected by in vitro 51Cr-release test or in vivo Winn's test were raised by immunization with Mitomycin C-treated EL-4 tumor cells in syngeneic C57BL/6J mice. Cytotoxicity of PE cells was found to reside in glass-nonadherent and anti-Thy 1.2-sensitive population, while such activity was not detected within the spleen and mesenteric lymph nodes. Intraperitoneal or oral administration of PSK augmented such resistance or cytotoxicity.