From bovine plasma globulins, "A" was prepared by adsorption on Hyflo (Johns-Manville). From the remaining globulins, converter and thrombin fractions were prepared by adsorption on barium sulfate, elution and ammonium sulfate precipitation. In a concn. of 0.02 [mu]g. protein-N per ml., the converter accelerated conversion of crude prothrombokinase. One [mu]g. protein-N per ml. accelerated activation of prothrombin, in the presence of "A." Larger quantities of converter fraction activated prothrombin, without addition of "A." The converter may be a thrombokinase that activates not only prothrombin, but also prothrombokinase. The ratio of converter activity to thrombin activity was 100 times as great in the converter fraction as in the thrombin fraction.