IFN-γ–null and wild-type mice exhibit the same acute and chronic responses to viral infection. Wild-type (+/+) and IFN-γ–null (–/–) mice were inoculated with SeV (5,000 EID₅₀) and analyzed as follows. (a) Body weights relative to initial values were determined as mean ± SEM of eight to ten mice per group. No significant difference was found for wild-type versus IFN-γ–null cohorts. (b) Lung sections from IFN-γ–null mice were immunostained with anti-SeV Ab and counterstained with hematoxylin on the indicated postinoculation days. Representative photomicrographs are shown for each genotype (five mice/genotype). Bar, 20 μm. (c) Airway reactivity to inhaled methacholine was assessed at the indicated times before and after inoculation with SeV (5,000 EID₅₀) or SeV-UV as described in Figure 3 legend. Values are shown for the final concentration of methacholine (160 mg/ml) and represent mean ± SEM of nine mice. The same pattern was observed at lower concentrations of methacholine, and the values for Penh in wild-type or IFN-γ^–/– cohorts that received no virus were no different from those for SeV-UV (data not shown). *Significant increase compared with control mice that received SeV–UV.