Production of interleukin 12 (IL-12) during cytotoxic reaction of C57BL/6 and BALB/c spleen lymphocytes or their subpopulations: T+ B, T, CD4+ and CD8+ cells to L. innocua-phagocyting syngeneic macrophages was examined. The effector cell donors were untreated or L. innocua-infected. The number of surviving bacteria in phagocytes was tested and IL-12 level in culture supernatants of reacting cells was determined. C57BL/6 mice, resistant to Listeria infection, were found to develop stronger cell cytotoxicity to bacteria-phagocyting syngeneic macrophages than BALB/c mice. The lymphocytes responsible for that phenomenon were of CD8+ phenotype. IL-12 was produced only during nonspecific cytotoxic reaction of CD4+ and CD8+ T cells. It is suggested that the innate resistance of mice to Listeria is dependent on their ability to develop a specific cytotoxic reaction and IL-12 production.