Current evidence indicates that factor VIII is a glycoprotein complex of two distinct proteins or polypeptide components, which circulates as polymers ranging in size from 800 000 to 20 × 106 daltons. Lack of synthesis of whole or part of this complex results in a bleeding diathesis. One protein (VIII:C) is controlled by an X-linked gene, the other (VIII:vWf) is inherited autosomally. The two major biological functions of this molecular complex are participation in coagulation and involvement in adhesion of platelets to damaged surfaces. Unlike most other proteins involved in coagulation, factor VIII is not a serine protease and its structure and amino acid composition is unknown.