An additional TaqI restriction site was mapped in intron 2 of the factor XII gene. The site was found only in subjects with total or partial factor XII deficiency and thus represents the true gene lesion or a very tightly linked restriction fragment length polymorphism. The altered gene identified by this marker is present in four (three heterozygotes and one homozygote) of five unrelated Hageman trait subjects from different Italian regions. In the homozygous state the altered gene gives rise to a very marked reduction of factor XII activity. No deletion was found in the deficient factor XII genes.