In viral myocarditis, we previously reported that antigen-specific T cells infiltrate the heart and play an important role in the pathogenesis of myocardial damage. For antigen-specific T-cell activation to occur, it is necessary for T cells to receive costimulatory signals provided by costimulatory molecules expressed on antigen-presenting cells as well as main signals provided by binding of T-cell receptors to antigens. To investigate the roles of costimulatory molecules B7-1 and B7-2 in the development of acute viral myocarditis, we first analyzed the expression of B7-1/B7-2 in the hearts of mice with acute viral myocarditis induced by coxsackievirus B3 (CVB3). Second, we evaluated the induction of B7-1/B7-2 in cultured cardiac myocytes treated with interferon gamma (IFN-gamma). Third, we examined the effects of in vivo administration of anti-B7-1/B7-2 monoclonal antibodies (mAbs) on the development of acute viral myocarditis. We found that CVB3-induced murine acute myocarditis resulted in enhanced expression of B7-1/B7-2 in cardiac myocytes. The expression of B7-1/B7-2 in cardiac myocytes could be induced in vitro by IFN-gamma. We found that in vivo anti-B7-1 mAb treatment markedly decreased myocardial inflammation, whereas anti-B7-2 mAb treatment abrogated the protective effect of anti-B7-1. Our findings indicate that distinct roles for B7-1 and B7-2 antigens are involved in the development of acute viral myocarditis and raise the possibility of immunotherapy with anti-B7-1 mAb to prevent T-cell-mediated myocardial damage in viral myocarditis.