Mice were first primed with a type A or a type B influenza virus and then challenged intranasally at least 1 month later with another type A virus. Potent cytotoxic T cell populations were found in lung, and effector T cell function was also demonstrated in blood, spleen and mediastinal lymph nodes. Lymphocytes isolated from all of these anatomical sites were active against target cells infected with the same, or with serologically different, type A influenza viruses. Also, prior exposure to another type A virus resulted in more rapid development of effector function than was seen in mice that had first been infected with B/Lee. Cytotoxic T cell populations generated in mice with influenza thus tend overall to be type-specific, and there is substantial localization of these effector lymphocytes in the pneumonic lung.