As reported earlier, an intraperitoneal injection of 1 mug of endotoxin (ET) from Serratia marcescens rendered mice resistant against the nonspecific mouse ascites tumor TA3-Ha upon challenge 24 h after pretreatment with ET. Further studies were aimed at the elaboration of conditions which achieved maximal resistance. It appears that (i) a 10-mug dose of ET was approximately the optimal dose for protection; (ii) pretreatment with ET 3 to 0 days prior to tumor challenge gave best protection; and (iii) the intravenous injection of ET showed a lower protection against the tumor than intraperitoneal application. Studies on the mechanism of ET protection indicate that (i) ET does not have a direct cytotoxic effect on tumor cells; (ii) normal spleen cells exposed to ET in vitro can adoptively transfer protection against tumor; and (iii) spleen cells activated in vivo by intravenous injection of ET can adoptively transfer protection. The possible involvement of mononuclear cells is discussed.