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.