The role of cell mediated immune responses in recovery of mice from influenza virus infection was studied by immunosuppression and by adoptive immunization. Virus infections persisted longer and produced more severe lung lesions in animals injected with anti-thymocyte serum (ATS), and immunosuppressed animals failed to mount a serum HI antibody response. Mice injected with ATS after the 4th day of infection produced antibody in titers equivalent to those of control animals but still did not recover from infection as rapidly. Passive immunization with antibody late in infection did not facilitate clearance of virus from ATS injected animals. Adoptive transfer of spleen cells obtained from syngeneic animals sensitized intraperitoneally resulted in more rapid clearance of virus and less severe lung lesions in infected recipient animals. This effect was associated with a lower antibody response in recipient mice. Adoptive immunization was still effective after depletion of beta lymphocytes from the transferred cell population. These effects correlate with in vitro assays of the cytotoxic T cell response and antibody forming cell response of sensitized mice. From these observations we conclude that T lymphocytes may play a role in recovery from influenza virus infection by mechanisms other than helper effects.