There is no excess of clot-inhibiting substance in hemophilic blood. Whether prothrombin is present in normal amount is uncertain. When the clotting time of hemophilics was reduced by inducing a local protein skin reaction, normally reacting prothrombin was recognized. The addition of peptone to hemophilic serum in vivo results in a reappearance and preservation of thrombin. Cephalin has an activating action on such serum-peptone mixtures.