Howell''s method was found to be most efficient method of preparation. Prothrombin so prepared was activated to thrombin by CaCl2, CHCl3, C2H5OH, and also by standing in aqueous solution. Prothrombin is a pseudoglobulin and as such can be fractionally precipitated by (NH4)2SO4. Prothrombin on dialysis divides into a water-soluble (normal thrombin) and a water-insoluble (fibrinolytic thrombin) portion. Normal thrombin contained no P whereas the fibrinolytic type did. A provisional hypothesis is made that prothrombin is a complex molecule consisting of these 2 complex thrombin groups; and the activation of prothrombin consists in the precipitation of one of these groups as modified thrombin (water-insoluble and fibrinolytic), the other remaining in solution as normal or P-free thrombin.