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We have investigated the pathway of prothrombin activation in blood and plasma. By means of a rapid purification procedure involving chromatography on DEAE-cellulose and hydroxyapatite, we demonstrated that the major prothrombin fragment in serum is that representing the amino-terminal half of prothrombin (i.e. F1-2). The F1-2 isolated was characterized by its size, amino acid and antigenic compositions, amino-terminal residue, and the peptides (designated F1 and F2, respectively) it yielded upon hydrolysis by thrombin. Measurements by the isotope dilution technique showed that F1-2 could account for the fate of at least 90% of the prothrombin originally present in plasma. By contrast, the serum concentration of the fragment representing the amino-terminal third of prothrombin (viz. F1) was less than 10% that of F1-2. These results demonstrated that the major route of prothrombin conversion in blood or plasma involves the removal of the combined activation fragment (F1-2) as a single peptide.