The blood-clotting protein prothrombin can be converted to thrombin in free solution by the proteolytic enzyme, activated factor X. When prothrombin is bound to the surface of phospholipid vesicles, the rate of thrombin generation is increased more than 30-fold over the rate of unbound prothrombin. If the prothrombin activation process is terminated after a time interval in which less than 10% of the expected thrombin has been produced, two major products are found in the activation mixture. These products have been termed intermediate 1, a precursor of thrombin, and fragment 1. The conversion of intermediate 1 to thrombin is not accelerated by phospholipid nor can binding of this intermediate to phospholipid particles be demonstrated. In contrast, fragment 1, the other activation product derived from prothrombin, binds to phospholipid particles under the same conditions as prothrombin. On the basis of these observations, we propose that the prothrombin molecule contains a specific region of polypeptide chain for binding to phospholipid particles. This specific polypeptide region or lipid interaction site is part of the nonthrombin-forming activation fragment.