The mechanism of action by which large doses of aprotinin decrease blood loss during cardiac surgery is not completely understood. In a prospective, controlled study, 30 patients undergoing cardiac surgery were given high-dose aprotinin in accordance with a commonly used regimen. Twenty untreated but otherwise comparable patients served as the control group. The effects of aprotinin therapy during cardiopulmonary bypass on coagulation parameters, the kallikrein-kinin system, fibrinolysis, platelet stimulation, and the release of elastase from neutrophils were studied. The fibrinolysis parameters were the only measurements that showed clear and significant differences between the two groups. Aprotinin almost completely inhibited the formation of fibrin and fibrinogen degradation products. It is assumed that inhibition of systemic fibrinolysis and suppression of local fibrinolysis contribute to the hemostatic action of aprotinin. The study did not demonstrate a significant protective effect of aprotinin on platelets. In addition, the dose of aprotinin administered did not affect the kallikrein-kinin system of elastase. Therefore, these data suggest that the previously demonstrated hemostatic effects of aprotinin derive primarily from its antifibrinolytic action.