The systemic infusion of thrombin-free human serum induces massive thrombosis in rabbit vein segments containing stagnant blood far removed from the site of infusion. In contrast, carefully collected human plasma is inert in this system. The data indicate that this phenomenon of in vivo thrombosis is quantitatively related to the amount of serum infused and forms the basis for a bioassay of the thrombosis-inducing activity. This bioassay, in providing a technique for detecting and quantitating the thrombosis-inducing activity of normal human serum, may facilitate separation of this activity from other recognized coagulation factors and serve as a guide to its ultimate isolation, purification and characterization. Note: (With the Technical Assistance of Marjorie Nickles, Mary-Lou Bloede, Bela J. Szalai and Rita J. Nickerson) Submitted on June 18, 1959

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SummaryEvidence has been obtained in rabbits that the reduction of the silicone whole blood clotting time to the normal glass range reflects a hypercoagulable state attributable to circulating activated factors XII and XI. The potential clinical implications of these findings are discussed.