The polymerase chain reaction (PCR) was used to amplify specific DNA sequences within the factor IX gene of haemophilia B patients and their relatives. Three of the amplified fragments contain polymorphic sites, which can be used as markers in segregation analyses. These restriction fragment length polymorphisms (RFLPs) were until recently detected by Southern blotting after digestion with the restriction enzymes Taq I, Dde I and Xmn I. All three RFLP's are located in introns of the factor IX gene and together are informative in approximately 70% of all cases. Each of the polymorphisms was successfully used in carrier detection studies after amplification of the relevant fragments. This method is also suitable for rapid antenatal diagnosis. Additionally we were able to amplify all eight exons of the factor IX gene including the splice junctions and a part of the 5'-region. Large deletions or insertions can be detected without further analysis. Several possibilities for the rapid detection of point mutations after DNA amplification have been described recently. The complete amplification of all functional parts of the Factor IX gene in combination with these new techniques should enable us to detect the majority of mutations leading to haemophilia B.