The interaction of aprotinin with normal coagulation was studied in blood samples obtained from 10 healthy subjects. Each sample was simultaneously tested in four different preparations: NaCl-treated blood: 0.03 mL 0.9% NaCl in 0.33 mL blood; aprotinin treated blood: 0.33 mL blood+aprotinin in 0.03 mL in aliquots to obtain a final blood concentration of respectively 50 KIU mL-1; 100 KIU mL-1 and 200 KIU mL-1. The coagulation process was analysed by thromboelastography. R-time, reflecting intrinsic coagulation, increased in a dose dependent manner between NaCl-treated and aprotinin-treated blood. These findings suggest a dose dependent impairment of intrinsic coagulation by aprotinin.