The effect of heat-killed Lactobacillus casei YIT 9018 (LC 9018), a potent antitumor agent as well as a potent macrophage-activating agent, on the resistance of mice to Listeria monocytogenes was studied. Mice pretreated intravenously (i.v.) with 0.5 mg of LC 9018 showed enhanced resistance to subsequent i.v. infection with L. monocytogenes. Antigenic cross-reaction between L. monocytogenes and LC 9018 was not seen in the delayed footpad reaction in mice, and LC 9018 inhibited the growth of L. monocytogenes in the liver and spleen of athymic nude mice. Therefore the antilisterial activity of LC 9018 was considered to be expressed nonspecifically and independently of the thymus. Carrageenan, a selective macrophage blocker, diminished the antilisterial effect of LC 9018. Peritoneal cells induced by intraperitoneal administration of LC 9018, which were composed mainly of macrophages and polymorphonuclear cells, showed enhanced phagocytosis and marked intracellular killing activity against L. monocytogenes in vitro as compared with those of the control. Kupffer cells of mice pretreated i.v. with LC 9018 produced a high oxygen radical level. The antilisterial effect of LC 9018 was maintained even at 21 days after its injection.