The complement (C) system-mediated neutrophil activation, adhesion to the coronary endothelium and accumulation into cardiac tissue are key steps in the pathogenesis of myocardial ischemia-reperfusion (MI/R) injury. We examined the differential role of the classical and the alternative complement pathway in MI/R injury in vivo. Rats were subjected to 20 min of myocardial ischemia followed by 24 h of reperfusion. Either a classical pathway inhibitor [C1 esterase inhibitor (C1-INH)(15 mg/kg)] or an alternative pathway inhibitor soluble complement receptor 1 (sCR1)[des-LHR-A](15 mg/kg) or their vehicle were administered intravenously 1 min prior to reperfusion, and myocardial necrosis (creatine kinase loss) and neutrophil accumulation, cardiac myeloperoxidase activity, were examined. C1-INH significantly attenuated cardiac creatine kinase loss compared to MI/R rats given only vehicle (p< 0.05) 24 h after reperfusion. An alternative pathway inhibitor, sCR1 [des-LHR-A] attenuated myocardial injury to a lesser extent, although it was not significantly different from the value for C1-INH or vehicle. Besides cardiac myeloperoxidase activity, the ischemic cardiac tissue was significantly attenuated by both C1-INH and sCR1 [desLHR-A](p< 0.05 vs. vehicle). Both the classical and alternative pathways may contribute to MI/R injury via a neutrophil-dependent mechanism in vivo. Selective inhibition of the classical pathway of complement activation seems to be slightly more effective in limiting necrotic MI/R injury than the selective alternative pathway inhibition in this 24 h model of reperfusion injury, but equal doses of each inhibitor attenuated neutrophil accumulation.