A comparison has been made of the electrophoretic behavior, chemical composition, amino-terminal sequence, and immunological properties of bovine prothrombin, factor IX (Christmas factor), and factor X (Stuart factor). Some immunological crossreactivity was found between the antibody to prothrombin and factor X although prothrombin and factor X differ substantially in amino-acid and carbohydrate composition. Considerable amino-acid sequence homology was found in the amino-terminal portion of prothrombin, factor IX, and the light chain of factor X. These data provide further evidence to support the hypothesis that at least three of the vitamin K-dependent clotting factors have evolved from a common ancestral gene.