Factor XIII consists of two catalytic a and two noncatalytic b subunits. The gene for the a subunit is located on chromosome 6, the gene for the b subunit on chromosome 1. Both genes have been characterized. There are several different allelic forms of the a subunit in the normal population and some microheterogeneity for the b subunit. Most patients with congenital factor XIII deficiency lack the a subunit in plasma; few patients appear to have a complete lack of the b subunit. The genes from patients with factor XIII deficiencies were obtained and examined. Based on these analyses a new genetic classification for factor XIII deficiency is proposed: a deficiency of the a subunit (formerly termed type II), a deficiency of the b subunit (formerly known as type I), and a possible combined deficiency of both a and b subunits.