Factor IX activity (IX:C) and antigen (IX:Ag) as well as factor VII, prothrombin time, Normotest and Thrombotest were determined in 37 haemophiüacs B from 26 kindreds, in 30 carriers of haemophilia B and in 40 healthy subjects. In 15 obligatory carriers from 9 kindreds, in which haemophiliacs did not exhibit IX:Ag in excess of IX:C, IX:C was parallel with IX:Ag, but in 4 from 4 kindreds, in which haemophiliacs exhibit excess of IX:Ag, there was a discrepancy between IX:C and IX:Ag. Among the obligatory carriers 6 from 3 kindreds, in which haemophiliacs did not have IX:Ag in excess of IX:C, showed that the anomaly M is not a sign of genetic variability of haemophilia B