We investigated how NK cells, extrathymic T cells, and thymus-derived T cells are activated in mice during infection with an intracellular pathogen, Listeria (L.) monocytogenes. Although macrophages and granulocytes are known to be involved in the elimination of this pathogen in an early phase of infection, it was still controversial what type of lymphocytes are induced as effectors in subsequent phases. When mice were ip injected with 1 x 10(3) L. monocytogenes (a sublethal dose), a prominent increase in the number of mononuclear cells in the liver and spleen was induced. Phenotypic analysis revealed that serial induction of lymphocyte subsets, NK cells-->extrathymic T cells-->thymus-derived T cells, occurred in these organs. Extrathymic T cells were estimated to have intermediate CD3 and a high level of IL-2 receptor beta-chain on the surface (i.e., intermediate CD3 cells). These mice became free from infection after 2 weeks. In the case of oral administration, 1 x 10(3) L. monocytogenes increased the number of cells in the liver and the number of intraepithelial and lamina propria lymphocytes in the intestine. Phenotypic analysis also showed a sequential induction of lymphocyte subsets in the liver and the induction of extrathymic T cells in the intestine. Preelimination of intermediate TCR cells and NK cells by in vivo treatment with anti-LFA-1 mAb made mice susceptible to an ip injected sublethal dose of L. monocytogenes. These results reveal a unique order of lymphocyte induction during listerial infection and indicate that extrathymic T cells might be one of the important cells in achieving resistance against L. monocytogenes.