Spleen cells from WKA rats immunized with allogeneic Donryu tumor (AH66), when examined by the Winn test, inhibited lethal growth of admixed antigenically unrelated syngeneic tumor (KMT-17) only when a mixture of spleen cells and tumor cells was injected into pre-irradiated recipients. On the other hand, spleen cells specificially sensitized to KMT-17 inhibited growth of admixed KMT-17 cells either in irradiated or non-irradiated rats.