Heparin has been used for more than fifty years for treatment of thromboembolism. However, there are significant limitations with the use of heparin including the inability of heparin to inhibit clot-bound thrombin and decreased efficacy in clinical situations where antithrombin concentration is reduced. Scientists have attempted to overcome these limitations by developing antithrombin-heparin covalent complexes with limited success. Recently, we developed a simple method to generate potent antithrombin-heparin covalent complexes, which can inhibit clot-bound thrombin. This product has been characterized for its functional and structural properties. Animal experiments have shown that covalent antithrombin-heparin is potentially useful both as a systemic anticoagulant and a surface coating