Anticoagulation during cardiopulmonary bypass (CPB) is necessary to prevent clotting as blood comes in contact with the unphysiologic surfaces of the extracorporeal circuit, and is achieved by administration of unfractionated heparin which binds to antithrombin III (ATIII) to potentiate and accelerate its inhibition of thrombin and other activated coagulation factors. In adults, heparin dose responses are decreased as ATIII concentrations fall. Findings in adults indicate that the administration of supplemental ATIII as concentrations fall below 80 u/dl may help to preserve heparin's effectiveness during CPB(1). Since ATIII levels in infants do not reach 80 u/dl until 3-6 months of age (2), one would likewise anticipate that heparin dose-response relationships would be affected. However, preliminary investigations in children less than one year of age show that there is no correlation between preoperative ATIII concentrations with the projected heparin dose-response (HDR) curve or with changes in the activated clotting time (ACT) value after heparin administration(3). The lack of correlation of ATIII levels with these heparin dose responses in infants raises important questions for pediatric patients regarding the mechanism by which heparin exerts its anticoagulant effects as well as the adequacy of heparinization during CPB.