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1. A new hemorrhagic disease characterized by a prolonged whole blood coagulation time caused by the delayed formation of thrombin is described. 2. The patient suffering from this disease was found to have normal concentrations of all previously described coagulation factors. 3. Because the coagulation defect could be completely corrected by the addition of tissue thromboplastin or platelet-poor hemophilic plasma and because tests for prothrombin and prothrombin conversion accelerators gave normal results and prothrombin utilization was impaired, it was concluded that the missing factor was concerned with the production of thromboplastin in the plasma. Accordingly it was named the plasma thromboplastin component (PTC). 4. The clinical and laboratory resemblance of PTC deficiency and hemophilia is striking. However, the two diseases can be distinguished by the fact that normal serum, the 45 to 50 per cent saturated ammonium sulfate fraction of normal plasma, the citrate eluate from barium sulfate, which has been mixed with normal plasma or serum, and hemophilic plasma correct PTC deficiency but are ineffective in hemophilia. On the other hand, while the 33 per cent saturated ammonium sulfate fraction of normal plasma, barium sulfate adsorbed normal plasma, Cohn's Fraction I and PTC-deficient plasma correct the hemophilic defect, they are ineffective in PTC deficiency. 5. A method for the partial purification and concentration of the PTC factor is described.

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