Résumé : Objectives: Evaluation of the effect of aprotinin on heparin monitoring tests, on heparin doses, and on perioperative blood loss in patients undergoing coronary artery bypass grafting. Design: Randomized, prospective open study. Setting: The study was performed in the departments of Anaesthesiology and Haematology of the Erasme University Hospital, Brussels. Participants: Twenty-six patients undergoing primary coronary artery bypass grafting. Interventions: Twelve patients received aprotinin, and 14 received no treatment. Measurement and Main Results: Aprotinin significantly reduced blood loss during coronary artery bypass surgery and was associated with a slight reduction in the amount of heparin administered. Activated coagulation time and activated partial thromboplastin time were prolonged by the addition of aprotinin. Activated coagulation time and activated partial thromboplastin time were poorly correlated with heparin assays. On the other hand, two other clotting tests designed to monitor heparin therapy, namely Titrarine (Stago, Asniere, France) and Heptest (Haemachem, St. Louis, MO), gave very good correlation with amidolytic heparin assays and can be used during extracorporeal circulation. Thrombin time showed a good correlation with amidolytic heparin assays after protamine administration and can be useful to detect residual heparin after heparin neutralization by protamine.