Cytotoxic T cells are present in the lungs and the bronchoalveolar washings of mice infected intravenously (i.v.) or intranasally (i.n.) with live influenza A/WSN virus. After i.v. injection, cytotoxic T cell activity in both spleens and lungs reaches a peak at 6 days when the level of infectious virus recovered from the lungs falls sharply and the mice do not die. If a lethal dose of virus is given intranasally, very high levels of virus appear rapidly in the lungs, and the development of lung consolidation follows slightly behind the appearance of cytotoxic T cells there. When a non-lethal dose of virus is given intranasally, lower levels of virus are found in the lung and the appearance of cytotoxic T cells is delayed. These results suggest that the cytotoxic T cells play a protective role if the level of virus in the lungs does not reach very high levels. After injection of antithymocyte serum, the subsequent level of cytotoxic T cell activity in the lungs was greatly reduced, suggesting that the T cells recovered in lungs had at an earlier stage been circulating cells. However, splenectomized mice develop high levels of cytotoxic T cell activity, after intranasal infection of mice, indicating that the spleen did not contribute substantially to the T cells recovered in the lungs.