The in vivo regulation of coagulation is mainly controlled by plasma inhibitors in which antithrombin III (AT III) plays an importance role. AT III is a glycoprotein which inhibits all serine proteases, except factor, VIIa, generated during the coagulation process. The proteases are inactivated by formation of an equimolecular complex and this reaction is greatly enhanced in the presence of heparin. A similar catalytic process could occur in vivo, involving heparin-like substances present on the surface of the surface of the endothelial cell. The physiological importance of AT III is clearly demonstrated by the high incidence of thromboembolic disease in patients with congenital AT III défficiency.