SummarySteric and functional changes of thrombin zymogen, due to the degrading action of thrombin, have been studied. A prothrombin complex of a high degree of activity was obrained from bovine plasma by adsorption on BaCO3. This preparation was further purified by ion exchange chromatography and converted with a small amount of thrombin into a modified form of zymogen. The difference between the unmodified and modified form, with respect to their reactivity, has been seen only in a biological type of activation with factor Xa-phospholipid-factor V-calcium complex. The modified form, when reacting as a concentrated substrate, or the diluted one, under conditions of the two-stage analysis, required several-fold higher concentrations of factor V. With factor Xa alone as activating enzyme, both forms of zymogen reacted in exactly the same manner. Gel filtration studies indicated a drastic change in shape or size of the altered zymogen molecule, as defined by Stokes radii.Factor X present in prothrombin complex preparations, by its frictional characteristic, obviously resembled the unmodified form of prothrombin. However, it was not affected by thrombin either in a functional way or in its molecular parameters. This indicates that, although present together in the same multiactivity preparation, factor X and thrombin zymogen do not form a molecular complex.