Th1-type immune responses, mediated by IL-12-induced IFN-γ, protect the host from most viral infections. To investigate the role of IL-12 and IFN-γ on the development of Coxsackievirus B3 (CB3)-induced myocarditis, we examined the level of inflammation, viral replication, and cytokine production in IL-12Rβ1-and IFN-γ-deficient mice following CB3 infection. We report that IL-12Rβ1 deficiency results in decreased viral replication and inflammation in the heart, while IFN-γ deficiency exacerbates CB3 replication. Importantly, decreased IL-1β and IL-18 levels in IL-12Rβ1-deficient hearts correlated directly with decreased myocardial inflammation. Because IL-1β and IL-18 were associated with myocardial inflammation, we examined the effect of TLR4 deficiency on CB3 infection and myocarditis. We found that TLR4-deficient mice also had significantly reduced levels of myocarditis, viral replication, and IL-1β/IL-18, just as we had observed in IL-12Rβ1-deficient mice. This is the first report that TLR4 influences CB3 replication. These results show that IL-12Rβ1 and TLR4 exacerbate CB3 infection and myocarditis while IFN-γ protects against viral replication. The remarkable similarities between the effects of IL-12Rβ1 and TLR4 suggest that these receptors share common downstream pathways that directly influence IL-1β and IL-18 production, and confirm that IL-1β and IL-18 play a significant role in the pathogenesis of CB3-induced myocarditis. These findings have important implications not only for the pathogenesis of myocarditis, but for other autoimmune diseases triggered by viral infections.