Hyperfibrinolysis during orthotopic liver transplantation (OLT) has been attributed to high plasma levels of tissue plasminogen activator (t-PA). This study investigated the contribution of urokinase plasminogen activator (u-PA) to hyperfibrinolysis and the effects of high-dose perioperative aprotinin (Trasylol) on fibrinolytic activation. Plasma samples were collected before, during, and after OLT in fifty five patients receiving either high dose aprotinin or placebo in a randomized double-blind trial. t-PA antigen and u-PA antigen and activity levels were increased preoperatively compared with normal controls (P < 0.05). Hyperfibrinolysis was seen during the anhepatic phase as shown by shortened euglobulin clot lysis times (ECLT) and an increase in D-dimer titers. t-PA levels peaked on reperfusion and fell at the end of the operation, and u-PA levels did not increase during OLT, but showed a decrease at the end of the operation. With aprotinin treatment, t-PA levels were lower on graft reperfusion than the placebo group (P < 0.05), but there was no difference in u-PA antigen or activity levels between groups. Fibrinolytic inhibition during OLT by aprotinin was demonstrated by prolonged ECLT (P < 0.05), reduced D-dimer levels (P < 0.05), and an increase in antiplasmin activity (P < 0.05). This study showed that the main antifibrinolytic action of aprotinin is as an antiplasmin agent with some effect on t-PA-but not u-PA-mediated fibrinolysis.