Twenty three patients belonging to 18 different pedigrees of Haemophilia B were studied with regard to ox-brain prothrombin time and its correlation to factor VII. Eleven among them were B-negative (no detectable factor IX antigen), five were B-reduced (factor IX antigen detectable but below the normal values) and seven were B-positive (normal levels of factor IX antigen). Ox-brain prothrombin time was found prolonged (greater than or equal to mean + 2.5 SD:99% confidence limits) in nine patients. Factor VII Activity (VII:C) was found reduced in 1/11 B-negative, in 2/5 B-reduced and in 4/7 B-positive patients. Factor VII Antigen (VII:Ag) was found normal in all but one patient. The ratio VII:C/VII:Ag was abnormal in eight patients independently from the variant of Haemophilia B. The underlying defect which causes the prolongation of Ox-brain prothrombin time due to factor VII:C mild deficiency is heterogeneous. Age, a mild Vitamin K deficiency, the presence of an inhibitor of Factor VII activation and other unknown causes, may be responsible for this pattern.