Genetic variation of the B subunit of human coagulation factor XIII has been observed after electrophoresis of plasma or serum samples on thin layer agarose plates and subsequent immunofixation with a specific antiserum. The F-XIIIB locus is autosomal and has three alleles. In Australian blood donors, the F-XIIIB1, F-XIIIB2 and F-XIIIB3 alleles have frequencies of .747, .084, and .169, respectively.