Since most thrombotic reactions occur on the vessel wall, interaction of anticoagulants with vascular components is critical. Heparin (H) is the primary drug for treatment and prevention of thrombosis. To improve Hs efficacy and bioavailability, a covalent complex of H and its biological target, antithrombin (AT), was developed. While H has a short, variable intravenous half-life leading to unpredictable anticoagulation, clearance of covalent ATH complex is slower. Hs variable anticoagulant effect arises from interactions with plasma and vessel wall proteins. ATH has increased bioavailability due to lower plasma protein and endothelial binding relative to H. Pharmacodynamic studies demonstrate that the AT moiety can regulate ATH binding to target tissues. For example, blood vessel binding is enhanced using ATH containing recombinant AT with oligomannose structures that can interact with endothelial mannose receptor lectins. Furthermore, recent work has shown that inhibition of factor VIIa/tissue factor complex by ATH is significantly faster than AT + H. Similarly, thrombin bound to endothelial thrombomodulin is inhibited more efficiently by ATH than by AT + H, which might improve regulation of thrombin generation. Overall, linking H to AT may prevent unwanted protein interactions and allow vessel wall sites to be targeted. This review examines ATH biodistribution.