Thirty-two families with haemophilia B were divided into sub-groups based on assessment of fIX:Ag, fIX:C and the thromboplastin time with bovine thromboplastin. In confirmed carriers from these families after assessment of fIX:Ag and, fIX:C the normal range of individual sub-groups of confirmed carriers was assessed and compared with values of fiX:Ag and fiX:C of the control group. It will be possible to use these groups after their extension to assess probable carriers in sub-groups CRM+ and CRM-. In the sub-group CRMR the only criterium of transmission is a reduced fIX:C level. A prolonged TP with bovine thromboplastin in families with BM haemophilia cannot be used to assess carriers.