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A newly recognized patient with the factor X Friuli coagulation disorder is presented. The propositus is a 10-year-old boy who presented excessive bleeding after a tooth extraction. This is the 11th patient with this peculiar coagulation disorder discovered in Friuli and the 12th so far described. His parents are consanguineous and heterozygotes together with other family members. The main laboratory features, typical of the disease were prolonged prothrombin time, a prolonged partial thromboplastin time, and a normal Stypven-cephalin clotting time. The incidence of the homozygotes in the Friuli area appear to be 0.000005 (11 cases in a population of 2,000,000). The probable frequency of the abnormal f gene is 0.0033, whereas that of the normal gene F is 0.9967. The theoretical frequency of the heterozygote, Ff condition is 0.007, namely 7 heterozygotes every 1,000 persons. However, only about 70 ascertained heterozygotes have been discovered so far.

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.