A normal human population has been screened for the existence of further restriction fragment length polymorphisms (RFLPs) in the clotting factor IX gene in addition to the TaqI polymorphism already characterised (1,2). Two polymorphic loci were found, both within 6 Kb of the TaqI polymorphism within the body of the factor IX gene. One of the polymorphisms has been shown to be due to either the presence or absence of a particular recognition site for the restriction enzyme XmnI. The other, visualised as a difference in fragment pattern produced by digestion with either HinfI or DdeI, has two allelic forms differing by a 50 bp element of inserted DNA. Sequence analysis has shown the inserted element to be in a region of Z type DNA sequence, the insertion representing a duplication of flanking sequence on either side. The two polymorphisms are inherited in simple Mendelian fashion and have both been used to diagnose haemophilia B carrier status. It is estimated that the combined use of these polymorphisms in the factor IX gene, despite linkage disequilibrium between the 3 polymorphic loci, should enable carrier status to be determined in approximately 66% of all haemophilia B families.