In order to prevent thrombosis we probably have to know how to prevent activation of prothrombin or to immediately inactivate thrombin. This requires that we can design and synthesize inhibitors that are either highly specific for thrombin (not attacking any other serine proteinases, like plasmin and the digestive enzymes) or for prothrombin making it unavailable to normal activation. To do this kind of work in even a vaguely intelligent fashion we shall have to know the structures of thrombin and prothrombin. The present project proposes to solve the primary structure of prothrombin (including thrombin) as a first small step in this direction.