Plasma concentrations of the complement products C3dg and the terminal complement complex, as well as the number of granulocytes (polymorphonuclear neutrophils), were assessed in patients undergoing aorta-coronary bypass with extracorporeal circulation, abdominal aneurysmectomy with implantation of an aortic graft, or thoracotomy without the introduction of synthetic material into the circulation. The concentration of terminal complement complex increased significantly only in the group undergoing extracorporeal circulation, with a corresponding drop in the number of granulocytes. In contrast, the C3dg concentration increased during both extracorporeal circulation and abdominal aneurysmectomy, which indicates that other factors than extracorporeal circulation may affect C3 activation during major operations. In the thoracotomy group, where the most pronounced increase in granulocytes was found, no complement activation was recorded. It is concluded that extracorporeal circulation activates the terminal pathway of complement and that assays detecting activation of both the initial and the terminal parts should be included when the pathophysiology of complement is examined during major operations and extracorporeal circulation.