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 -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 the growth factor domains found in several other plasma serine proteases. Thus far, no function has been ascribed to these regions, but there is suspicion that they may play a role in one of several binary protein interactions involving prothrombin. The mature single chain protein circulates in plasma as a zymogen and, during coagulation, is proteolytically activated to the potent serine protease -thrombin. This proteolysis is catalyzed by the prothrombinase enzyme complex. During activation, human prothrombin is cleaved at Arg273-Thr274 and at Arg323-Ser324 to yield a “pro” fragment (fragment 1.2) and thrombin, the latter of which is composed of two chains covalently linked by a disulfide bond. Human prothrombin is prepared from fresh frozen human plasma as described by Bajaj and coworkers (2). Bovine prothrombin is prepared from fresh bovine plasma using a modification of the procedure described by Owen and coworkers (3). Mouse prothrombin is prepared from fresh frozen murine plasma using conventional chromatography techniques. Purified prothrombin is supplied in 50% (vol/vol) glycerol/H2O and should be stored at -20°C. Purity is determined by SDS-PAGE analysis, and activity is measured by clotting and/or chromogenic substrate assay, following conversion of prothrombin to thrombin.