Influenza A virus causes recurring seasonal epidemics and occasional influenza pandemics. Because of changes in envelope glycoprotein Ags, neutralizing Abs induced by inactivated vaccines provide limited cross-protection against new viral serotypes. However, prior influenza infection induces heterosubtypic immunity that accelerates viral clearance of a second strain, even if the external proteins are distinct. In mice, cross-protection can also be elicited by systemic immunization with the highly conserved internal nucleoprotein (NP). Both T lymphocytes and Ab contribute to such cross-protection. In this paper, we demonstrate that anti-NP IgG specifically promoted influenza virus clearance in mice by using a mechanism involving both FcRs and CD8(+) cells. Furthermore, anti-NP IgG rescued poor heterosubtypic immunity in B cell-deficient mice, correlating with enhanced NP-specific CD8 T cell responses. Thus, Ab against this conserved Ag has potent antiviral activity both in naive and in influenza-immune subjects. Such antiviral activity was not seen when mice were vaccinated with another internal influenza protein, nonstructural 1. The high conservation of NP Ag and the known longevity of Ab responses suggest that anti-NP IgG may provide a critically needed component of a universal influenza vaccine.