An extended polymorphism of the coagulation factor XIIA can routinely be detected in human plasma samples and white cell lysates by isoelectric focusing in polyacrylamide gels containing 3 M urea in the pH range 5–8. Analyses of 184 families with 513 children confirmed the formal model proposed by Suzuki et al. [Am J Hum Genet 1988;43:170–174]. Four common alleles, F XIII*1A, 1B, 2A, 2B, at an autosomal locus control the expression of ten phenotypes. On the basis of the population sample from southwest Germany the frequencies of the common alleles F XIIIA*1A, 1B, 2A, 2B were calculated as 0.175, 0.609, 0.011, and 0.205, respectiv