Purity ≥95% by SDS-PAGE. Human prothrombin is prepared from fresh frozen human plasma as described by Bajaj, et al. (2). Purity is determined by SDS-PAGE analysis, and activity is measured by clotting and/or chromogenic substrate assay, following conversion of prothrombin to thrombin. Prothrombin is a vitamin K-dependent plasma protein which is synthesized in the liver (1). Prior to secretion into plasma, prothrombin undergoes post-translational modification by a vitamin K-dependent carboxylase which converts ten specific glutamic acid residues to g-carboxyglutamic acid (gla). The ten gla residues are located within the first 40 amino acids of the mature protein and contribute to the ability of prothrombin to bind to negatively charged phospholipid membranes. Prothrombin contains two regions of internal homology which are referred to as kringle structures. These regions of conspicuous secondary structure are located between residues 40 and 270 of the mature plasma protein and replace