The terminal, membrane-damaging complement complex C5b-9 accumulates in the infarcted myocardium. In experimental myocardial infarction, we investigated the time course of C5b-9 deposition and the influence of reperfusion. In a group of 17 rabbits (group 1), the circumflex coronary artery was occluded for different time periods ranging from 0.5 to 29 h without subsequent reperfusion. A second group of 23 rabbits (group 2) underwent coronary artery occlusion for periods ranging from 0.5 to 6 h followed by reperfusion. C5b-9 was determined in transmural myocardial biopsies by immunohistochemistry and by ELISA. In group 1, C5b-9 accumulation in the ischaemic myocardium was found only after 5 to 6 h of coronary artery occlusion. In group 2 (ischaemia and reperfusion), significant C5b-9 deposition was already observed after 30 min of myocardial ischaemia. We conclude that in the absence of reperfusion C5b-9 accumulation occurs as a late event when most of the jeopardized myocardium has probably already become necrotic. In the presence of reperfusion, however, the complement system is activated rapidly and this could play a role in the pathogenesis of reperfusion injury.