1. The presence of biological and immunological activity of prothrombin in sonicates from rough and smooth microsomes has been investigated. 2. Biological activity of prothrombin was detected in both the rough and smooth microsomal fraction. The specific and the total activity of prothrombin in the sonicates from smooth microsomes were 3-4-fold higher than in the corresponding fraction from rough microsomes. 3. Prothrombin could be identified in both microsomal fractions by double immunodiffusion. 4. The presence of a macromolecular inhibitor of blood coagulation in the microsomes is reported.