Interferon-γ has well-documented antiviral and immunomodulatory activity, but its role in the control of cytomegalovirus (CMV) infection is not well studied. In a mouse model of murine CMV (MCMV) disease, interferon-γ concentrations in serum but not in bronchoalveolar lavage fluid increased in response to viral infection. Serum interferon-γ levels peaked at day 2 in the relatively resistant C57BL/6 mice, and, in contrast, did not peak until day 6 in susceptible BALB/c mice. Mice genetically lacking interferon-γ (GKO) were more susceptible to MCMV, although strain differences persisted, with C57BL/6 GKO mice experiencing less severe MCMV disease than BALB/c GKO mice. Treatment of MCMV-infected BALB/c mice with exogenous interferon-γ starting 2 days after viral infection had a modest protective effect at lower interferon-γ, doses (104 units), but interferon-γ therapy markedly increased morbidity and mortality when higher doses (105 units) were used. We conclude that interferon-γ plays a significant role in host response to MCMV and that the cytokine has dose- and time-dependent beneficial and adverse effects.