In a prospective randomized double-blind study, the activated clotting time (ACT), heparin use, parameters of anticoagulation, and thrombin activation during extracorporeal circulation were studied in 20 patients who underwent aortocoronary bypass operations. The patients were divided into two groups: Group A was given a placebo, while Group B was given aprotinin according to the high-dosage Trasylol scheme. During ACT-controlled heparinization (ACT > 460 s) there was a significant heparin reduction in Group B (22,100 USP-E) in comparison to Group A (35,200 USP-E). Despite this lower quantity of total heparin, the ACT in Group B was significantly extended (Group B = 837 s, Group A = 492 s). The ACT did not correlate thereby with the heparin concentration or the total quantity of heparin in either group. In contrast to the control group, there was no increased thrombin generation in the aprotinin group. The thrombin-antithrombin III complexes (Group A = 143 micrograms/L, Group B = 102 micrograms/L) as well as the specific dimers (Group A = 2755 ng/ml, Group B = 448 ng/ml) were significantly lower under the use of aprotinin. The connection between the ACT, the heparin concentration, and the aprotinin concentration was further investigated in an ex-vivo model. The ACT samples were diluted with the aim of eliminating the influence of aprotinin. Under these conditions it was shown that for heparin concentrations between 2-4 U/ml there was a parallel shift of the ACT/heparinconcentration curve with the addition of aprotinin in a defined concentration range of 200-300 KIU.(ABSTRACT TRUNCATED AT 250 WORDS)