In the clotting of blood, Factor VIII is a labile protein of the blood-clotting system that assists in the conversion of Factor IX into plasma factor X (Stuart factor). Deficiency of factor VIII is associated with classic hemophilia A, a hereditary, sex-linked, hemorrhagic tendency that occurs almost exclusively in men; clotting time is prolonged, less thromboplastin is formed, and the conversion of prothrombin is diminished.