In full term and preterm infants the coagulation system begins to develop gradually during gestation and continues to mature toward the adult system during the postnatal period. By 6 months of age, most components of the coagulation system, both procoagulants and inhibitors, are well within the normal adult range(8). However, at birth the coagulation system is quantitatively immature with significant deficiencies in many factor levels. One of these deficient factors is AT III. AT III levels are only 60% of adult values at birth and do not attain adult levels until 3 to 6 months of age. In the face of these low AT III levels, the ability of heparin to produce adequate anticoagulation during cardiac surgery in infants becomes a significant concern, and it is reasonable to consider that there are potential differences in coagulation between neonates and adults undergoing CPB. Questions arise as to the role that diminished AT III levels have in producing an anticoagulant effect in young infants. Further questions arise as to the adequacy of anticoagulation during cardiac surgery in young infants since, the accepted standard of care, only ACT measurements, are routinely used to make this determination. The first question can be addressed by the comparison of AT III levels as well as other potential heparin cofactor levels with more specific indicators of the inhibition of the coagulation system. The second question can be addressed by the measurement of specific breakdown products released during the formation of thrombin upon activation of the coagulation system.