1. The evolution of a factor in serum which accelerates prothrombin conversion to thrombin has been studied in normal subjects.

2. Mechanical agitation of fresh blood or the addition of thromboplastin supplements increases the amount of SPCA evolved and decreases the amount of prothrombin activity remaining in the serum.

3. Retarding coagulation by large doses of heparin or by handling the blood with siliconized apparatus decreases SPCA evolution and increases residual serum prothrombin activity.

4. Hypoprothrombinemic blood resulting from dicumarol administration evolves subnormal amounts of SPCA during its coagulation.

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