Data are reported pertaining to the 57 subjects so far found to be heterozygotes for the abnormal factor X (factor X Friuli) coagulation disorder. 41 of these subjects were related to the 11 patients known to be homozygote for the defect. Their average factor X level was 59.7%. The difference between such an average and those found in the homozygote population (10.9%) and in two normal groups (106 and 103%) was statistically significant. The heterozygote usually presents a 1-to 2.5-sec prolongation of the prothrombin time. A significant negative correlation was found to exist between the prothrombin time prolongation in seconds and the percentile factor X level. Factor X in the heterozygote population results to be lower when assayed using human brain or human placenta thromboplastins as compared with a rabbit brain and lung thromboplastin. The cross-over electrophoresis mobility of heterozygote Friuli plasma is identical to that of homozygote Friuli plasma and normal plasma. About one third of the heterozygote patients presented bleeding manifestations.

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A new case with the abnormal factor X (factor X Friuli) coagulation disorder is presented: a 32-year- old female who was born outside Friuli and who complained of a bleeding tendency since childhood. The main laboratory features are prolonged prothrombin time, prolonged partial thromboplastin time, abnormal thromboplastm generation and normal Stypven-cephalin clotting time. The prothrombin time and the thromboplastin generation were corrected by the addition of normal serum. Factor X was low only when assayed using whole tissue or partial thromboplastin. It was normal using the Stypven-cephalin mixture. The cross-over electrophoresis showed a normal factor X band in the patient’s plasma, whereas no band was present in Mr. Stuart’s plasma. The father and a cousin of the patient are heterozygotes for the abnormality. This is the first family reported to have the abnormal factor X (factor X Friuli) disorder outside Friuli.

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An immunological study of factor X Friuli was carried out using an anti-human-factor X antiserum. The results obtained with different methods were compared. Only an ill-defined factor X band or precipitate was evident on standard immuno-electrophoresis and on immunodiffusion. Using a non-absorbed antiserum a precipitate was evident on eletroimmunoassay, in normal, factor X Friuli and in factor X deficient plasma. On the contary, using an antiserum which had been previously absorbed with factor X deficient plasma, a precipitate was evident only with factor X Friuli plasma or normal plasma. Excellent results were obtained with the cross-over electrophoresis. Using this method a major or factor X band was evident in normal plasma, factor II,P FACTOR VII, factor IX deficient plasmas and in the factor X FRIULI PLASMA. No factor X band was evident in factor X deficient plasmas and in absorbed normal plasma. Lighter, more cathodic bands were seen in all plasmas. These were due to secondary activities of the antiserum and could be disregarded. Friuli serum factor X and normal serum factor X showed a slightly more anodic migration as compared to their plasma counterparts. In coumarin plasma two factor X bands were evident; one normal in position and one slightly more catholidic. In the bidimensional immunoelectrophoresis two precipitates are visible in normal plasma and in factor X Friuli plasma. In factor X deficient plasma only one precipitate is evident. This latter precipitate is due to the secondary activity of the antiserum used. On the basis of these data it is concluded that factor X Friuli behaves, immunologically, as normal factor X.