We examined the intra- and postoperative behavior of antithrombin III (AT), factor V, VIII, and fibrinogen in 27 elective surgical patients without evidence of disseminated intravascular coagulation (DIC) and treated according to the concept of blood component therapy inaugurated at our hospital in 1975. The intraoperative depletions of AT and fibrinogen were proportional to the transfusion volumes and correlated significantly. AT, fibrinogen, and especially factor V and VIII were significantly mobilized during surgery. A greater intraoperative depletion of AT was significantly associated with a faster recovery during the first 24 postoperative h. The AT activity was virtually stable over a period of 4 weeks in CPD-adenine red cell concentrates ; fresh frozen plasma and whole blood are thus not essential as a source of AT. The application of our concept did not increase the frequency of thromboembolic complications, despite the fact that the intraoperative AT values fell below the presumed ‘critical’ level of 60-70% in some patients. The probable reasons are the brief duration of such levels, the simultaneous depletion of coagulation-promoting plasma constituents (e.g. fibrinogen), and the use of antithrombotic prophylaxis. Our results suggest no reasons for a routine use of fresh frozen plasma in patients with a loss and replacement of less than about 75% of their blood volume.