Components of the complement system are known to play an important role in the cytolytic process and in chemotaxis of leukocytes. Cobra venom factor specifically cleaves C3 activity via activation of the alternative (properdin) complement pathway. It does not act directly on C3. If C3 is involved in tissue necrosis after ischemic injury, cobra venom factor might reduce tissue damage after acute coronary occlusion. Accordingly, in 14 control dogs occlusion of the left anterior descending artery was carried out for 24 h. Epicardial electrograms were recorded 15 min after occlusion, and 24 h later transmural specimens for creatine phosphokinase activity (CPK) and for histological analysis were obtained from the same sites. In another 14 experimental dogs, 20 U/kg cobra venom factor was given intravenously 30 min after occlusion. Serum complement levels fell within 2-4 h to <20% of normal. In the control dogs, the relationship between ST-segment elevation and CPK activity 24 h later was: log CPK = -0.06 ST + 1.48 (n = 111 specimens, 14 dogs, r = 0.77). In the experimental dogs, log CPK = -0.024 ST + 1.46 (n = 111 specimens, 14 dogs, r = 0.60), showing significantly different slopes (P < 0.001), i.e., less CPK depression for any level of ST-segment elevation. Histologically, 69 of 71 sites (97%) with ST-segment elevation exceeding 2 mV in the control dogs showed signs of necrosis 24 h later, whereas in the experimental group only 43 of 79 sites (54%) with abnormal ST-segment elevations showed signs of necrosis (P < 0.0005). At the same time, it was shown that the administration of cobra venom factor did not alter cardiac performance, collateral blood flow to the ischemic myocardium or the clotting system, but infiltration of polymorphonuclear leukocytes into the myocardium was decreased. It is concluded that cobra venom factor, by reducing the amount of C3 and C5 substrate available for chemotactic factor generation, or other as yet undefined mechanisms, protects the ischemic myocardium from undergoing necrosis, as judged by histology and local CPK activity. Hence, a new approach to limiting the extent of myocardial infarcts after experimental coronary occlusion, based upon inhibition of complement-dependent inflammatory processes, is demonstrated.