Th1 cells control their activity by producing regulatory IL-10. Here we report that Th1 cell-derived IL-10 facilitates their expansion and, in addition, augments Th1 cell production of IFN-γ, TNF-α and IL-2 during the early phase of influenza. In our antigen-specific mouse experimental system, influenza haemagglutinin-specific CD4(+) T cells respond to infection with the induction of T-bet, and produce both IFN-γ and IL-10. In the early phase of infection, an abundance of viral neuraminidase causes TGF-β activation of haemagglutinin-specific CD4(+) T cells. CD4(+) T-cell-derived IL-10 inhibits neuraminidase-driven TGF-β activation and counteracts the virus-mediated immune suppression. As the host eradicates the virus, neuraminidase activity wanes and IL-10 receptors are upregulated on CD4(+) T cells in the late phase of infection. IL-10 then suppresses immune activation and aids in recovery from infection and inflammation. These results reveal a previously unrecognized function of Th1 cell-derived IL-10 in vivo.