Factor XI is a plasma glycoprotein which circulates in a non-covalent complex with high molecular weight kininogen. The mature molecule is synthesized in the liver and is a two-chain homodimer with a molecular weight of approximately 160,000. It is estimated that 5% of the total mass is attributable to carbohydrate (2). The two identical monomers have molecular weights of 80,000, and are joined together by disulfide bonds. Thus by SDS-PAGE analysis, factor XI appears as a single band both non-reduced (Mr=160,000), and reduced (Mr=80,000).