THE DISCOVERY of a case of congenital hypoprothrombinemia in 1941 by Rhoads and Fitz-Hugh 1 marked the beginning of an important advance in the study of hemorrhagic diseases. This case had been diagnosed and regarded as hemophilia for nine years. Only after the prothrombin-time test became available was it possible to differentiate such a condition readily from hemophilia. From the early studies * it became evident that cases of this type were congenital and likely familial. All were tacitly accepted as true hypoprothrombinemia. The discovery of the labile factor (proaccelerin, Factor V, or ac-globulin) in 1943 clearly demonstrated that the prothrombin time could be prolonged by a deficiency of this new factor, and the prediction that such a lack might cause a hemorrhagic disease 9 was borne out when, less than two years later, Owren 10 reported a patient with a bleeding tendency who lacked this agent. Shortly thereafter, members of