The effect of aprotinin, a protease inhibiting agent, on blood product requirements in liver transplantation was investigated in a prospective, open, non-randomized study. From September 1988 until January 1990, 60 consecutive liver transplantations were performed in 56 adult recipients. The first 10 patients (group I) were treated without aprotinin; for the next 50 transplantations (group II) aprotinin was applied in a total dose of 1.5-2.0 mio units i.v. intraoperatively. During primary transplants a mean of 9.7 +/- 5.5 units packed red blood cells (RBC) were substituted in group I vs. 7.5 +/- 4.6 units RBC in group II (n.s.). Fresh frozen plasma was substituted with 10.4 +/- 6.2 (group I) vs. 9.1 +/- 6.7 (group II) units (n.s.). Adverse effects of aprotinin were not observed. The trend towards decreased transfusion requirements and the observation of reduced fibrinolysis prompted us to continue routine use of aprotinin in liver transplantation.