Interferon-γ (IFNγ) is important in regulating the adaptive immune response, and most current evidence suggests that it exerts a negative (proapoptotic) effect on CD8⁺ T cell responses. We have developed a novel technique of dual adoptive transfer, which allowed us to precisely compare, in normal mice, the in vivo antiviral responses of two T cell populations that differ only in their expression of the IFNγ receptor. We use this technique to show that, contrary to expectations, IFNγ strongly stimulates the development of CD8⁺ T cell responses during an acute viral infection. The stimulatory effect is abrogated in T cells lacking the IFNγ receptor, indicating that the cytokine acts directly upon CD8⁺ T cells to increase their abundance during acute viral infection.