Another coagulation inhibitor, heparin cofactor II (HCII), forms a stable enzyme-inhibitor complex that inhibits thrombin in a reaction enhanced by heparin(4). Although in the adult population, the exact role of HCII is unclear, perhaps in neonates its role is more significant. Alpha-2-macroglobulin (2M), a less important inhibitor of thrombin in adults, has concentrations well above adult values in newborns and continues to increase in concentration during the first 6 months of life(2). In neonates, 2M has been shown to have an important role as a thrombin inhibitor although its association with heparin responses during CPB is unknown.