Purity ≥95% by SDS-PAGE. Human Factor XI is a plasma glycoprotein which circulates in a non-covalent complex with high molecular weight kininogen (1). The mature molecule is synthesized in the liver. It is a two-chain homodimer with a molecular weight of ~160kD (2,3). It is estimated that 5% of the total mass is attributable to carbohydrate (2). The two identical monomers have molecular weights of 80kD. They are joined together by disulfide bonds. By SDS-PAGE analysis, factor XI appears as a single band both non-reduced (Mr 160kD) and reduced (Mr 80kD). Factor XI circulates as a zymogen. It requires proteolytic activation to acquire serine protease activity. The conversion of factor XI to factor XIa is catalyzed by factor XIIa. This results in cleavage of the Arg369-Ile370 bond in each monomer (3). Factor XIa consists of two NH2-terminal derived heavy chains and two COOH-terminal derived light chains. All of which are held together by disulfide bonds. Factor XIa participates within the