NK cell functions for regulation of T cell responses were evaluated during acute viral infections. In vivo depletion studies established that the presence of NK cells in murine cytomegalovirus (MCMV)-infected immunocompetent mice negatively affected CD4 and CD8 T cell IFN-gamma expression, bromodeoxyuridine (BrdU) incorporation, and expansion. To evaluate NK cell effects, under conditions when NK cells do not control viral replication, experiments were performed using lymphocytic choriomeningitis virus (LCMV). Depletion of NK cells did not affect LCMV-elicited T cell responses in immunocompetent mice; however, the presence of NK cells did inhibit CD4 T cell IFN-gamma production, BrdU incorporation, and expansion in infected MHC class I- and CD8 T cell-deficient beta2M-/- mice. Together, the results reveal a previously unappreciated immunoregulatory role of NK cells for downstream T cell responses.