Direct carrier testing was done in 54 at-risk female relatives of haemophilic patients by initially analysing 2.46 kb of the factor IX gene in 1 haemophiliac per family by genomic amplification with transcript sequencing. A presumptive mutation was found in all 14 haemophiliacs examined. Analyses were then done either by sequencing the appropriate region in at-risk female relatives or by detection of an altered restriction site. A simulation indicated that the mutation will be associated with an altered restriction site in about half the families. The technique has clinical application.