In order to demonstrate whether specific cytotoxic T cells could be induced in lung parenchyma, C57BL/6 mice were immunized by the intrapulmonary route with allogenetic tumor cells (P815). Ten days after administration of 20 x 10(6) allogeneic cells, peak concentrations of cytotoxic cells were found in lung, tracheobronchial lymph node, and spleen. With reduction in immunizing dose, lytic activity disappeared from spleen and lymph node, but persisted in lung. The cytolytic activity was specific for the immunizing alloantigen, was abolished by antitheta serum, and could not be attributed to macrophages. For comparison, C57BL/6 mice were immunized by the intraperitoneal route with 20 x 10(6) P815 cells. The expected cytolytic activity was found in spleen and lymph nodes: however, unexpectedly high levels of cytolytic activity were also found in pulmonary lymphocytes. This activity was confirmed using a wide range of effector to largest cell ratios in the assay system. Quantitative cytolytic assays demonstrated that the maximum rate of cytolysis by pulmonary lymphocytes obtained from mice immunized intraperitoneally exceeded by 10- to 20-fold the rate of cytolysis by pulmonary lymphocytes obtained from mice receiving intrapulmonary immunization. These data demonstrate that cytolytic T-lymphocytes appear in lung parenchyma after either intrapulmonary or intraperitoneal immunization and that the intraperitoneal route is far more efficient than the intrapulmonary route. This cell-mediated immune mechanism potentially is available for host defense of respiratory tissue.