1. Under certain conditions crude prothrombin changes to thrombin without the addition of extraneous activators and in the absence of ionic calcium. 2. Thrombin is soluble in 0.45 saturated ammonium sulfate, whereas crude prothrombin is not. 3. Partially purified thrombin is comparatively stable in concentrated ammonium sulfate solutions at pH 5.2. 4. A method based on the above facts yields a thrombin preparation the specific activity of which is 100 to 175 times the potential specific activity of whole plasma.