1. The reported cases of idiopathic hypoprothrombinemia are reviewed briefly, and a case observed for over three years is presented. Particular attention is called to the similar clinical pattern presented by the chronic cases.

2. Studies are presented indicating that in this patient the delay in prothrombin time was due, at least in part, to a deficiency of a factor necessary for rapid conversion of prothrombin. This factor, or factors, which we have called Ac-globulin, is contained in a highly active state in fresh normal serum.

3. After the in vitro demonstration of a deficiency of Ac-globulin in the patient’s blood, it was possible to bring about a marked reduction in the patient’s prothrombin time by the intravenous administration of relatively small amounts (15 to 45 cc.) of fresh normal (thrombin-free) serum. A further reduction of the prothrombin time to near normal values was brought about by combined whole blood and serum administration. The evidence suggests that partial correction of both prothrombin and Ac-globulin deficiency respectively resulted from such therapy.

4. The possible effects of serum and whole blood upon the delayed prothrombin conversion rate of dicoumarolization and liver disease are discussed and preliminary observations in the former type suggest that such therapy may be useful.

ACKNOWLEDGMENTS We wish to express our appreciation to the following for their cooperation in this study: Dr. McLemore Birdsong, Associate Professor of Pediatrics, University of Virginia Medical School; The Department of Pediatrics, University of Virginia Hospital; The Department of Biochemistry, University of Virginia Medical School; Dr. Myers H. Hicks, Assistant Resident in Medicine, University of Virginia Hospital; Dr. Walter H. Seegers, Professor of Physiology, Wayne University College of Medicine.