Meth A tumor-bearing mice were examined for changes in the host defense mechanism against infection with Listeria monocytogenes. The resistance of tumor-bearing mice to systemic, i.e., intravenous, infection in an early phase of the infection was suppressed for 1-3 days after tumor implantation (5 x 10(5) cells/mouse, subcutaneously), but was augmented thereafter even on the 35th day as compared with normal mice. Suppression and enhancement of the resistance of tumor-bearing mice to primary infection with L. monocytogenes was also observed in tumor-bearing athymic nude mice. Splenic macrophages from tumor-bearing mice on the 14th day after tumor implantation exerted potent bactericidal activity against L. monocytogenes in vitro as compared with those from normal mice. The function of macrophages as nonimmune scavenger cells seemed to be activated in mice bearing a progressing tumor. However, some of the tumor-bearing mice challenged with a sublethal dose of L. monocytogenes showed diminished resistance in the late phase of the infection; moreover listeria-immunized tumor-bearing mice showed less resistance to a secondary challenge with the bacteria than did normal immunized mice. This suppression of the specific immune response of tumor-bearing mice to L. monocytogenes was shown also in the assay of the delayed-type footpad reaction. The bacterial growth-inhibiting function of listeria-immune T lymphocytes, determined by the effect of adoptive transfer of the cells on the growth of Listeria, was also reduced in tumor-bearing mice as compared with normal mice.