Prothrombin has been assayed by means of 7 different techniques in the patients so far known to have the abnormal factor X (factor X Friuli) coagulation disorder. The methods were: classical one-stage method, Iowa two-stage method, Oxford two-stage method, a modified Hjort’s method using Stypven-Cephalin, a modified Jobin and Esnouf’s method using Tiger venom, the Staphilocoagulase method and an immunological method with anti-human prothrombin rabbit serum. In all cases a normal prothrombin level was found. In the two-stage procedures a normal thrombin activity was obtained only when normal serum was added to the system. A good correlation was found between the several methods used. In subjects known to be heterozygote for the abnormal factor X coagulation disorder, prothrombin level and activity has been found to be normal too. These data suggest that the abnormal factor X (factor X Friuli) coagulation disorder is due to a distinct factor X protein anomaly unaccompanied by detectable changes in the prothrombin level or activity.