Studies were undertaken to characterize the lympho-proliferative response to primary influenza virus infection and to determine the specificity of antigenic recognition by proliferative T cells obtained from mice infected with influenza virus. A proliferative response to A/Port Chalmers/1/73 virus (H3N2) was first detectable in spleen cells 5 days postinfection. Activity persisted in immunized mice for at least 12 months. The duration of immune responsiveness detectable in the 3H-thymidine incorporation assay contrasts with previously reported data indicating a relatively transient appearance of splenic T cell immunity detectable by 51Cr release cytotoxicity assays on influenza-infected target cells. These experiments clearly indicate that splenic T cells from mice infected with influenza virus proliferate in vitro when exposed to antigens of the infecting virus and that they can distinguish between A and B type influenza viruses. The proliferative T cell recognition of influenza viruses within the A type, however, appears to have a broad cross-reactivity, in contrast to the specificity previously shown by cytotoxic T cells. Data obtained from A/Port Chalmers/1/73 virus treated with various anti-influenza sera suggest that the hemagglutinin glycoprotein is of considerable importance in the antigenic recognition by, and stimulation of, influenza-immune proliferative T cells. These assays of the T cell responses to influenza infection should be of value in investigating the complex immune response of the host to influenza viruses.