The matrix (M) protein of influenza A virus, one of the two group-specific internal proteins of the virion, was isolated in a pure form, and its immunogenicity was stable to heating at 100 degrees C for 2 min. Mice immunized with isolated M protein in complete Freund adjuvant and subsequently infected with influenza virus cleared virus more rapidly from their lungs than did unimmunized mice. Despite the rapid clearance of virus, the mice developed pneumonia that was at least as severe as in unimmunized mice. Preliminary studies suggest that the rapid clearance of influenza virus from the lungs of mice immunized with M protein may be initiated by a cell-mediated rather than a humoral response. The mechanism by which a cross-reactive internal virion protein can initiate clearance of the different subtypes of influenza is not clear. Perhaps the M protein is exposed on the surface of the virus-infected cell and is responsible for the cross-reactivity at the cytotoxic T-cell level recently detected between influenza A virus subtypes.