A 32-year-old woman with a bleeding tendency born of a consanguineous marriage, was found to have factor XIII subunit B deficiency. An abnormally low level of factor XIII activity was initially noticed and this finding led to further studies of the proband and her family. The notable features were: undetectable subunit B of factor XIII in the proband and her brother and reduced levels of subunit B, 34-52%, in her parents and children. The proband's brother had a markedly decreased level of subunit A protein. The level of factor XIII subunit A in platelets of the proband was normal. The half-life of subunit A determined from the disappearance curve of infused factor XIII subunit A concentrate was approximately 3 d and this is the shortest estimate of the half-life of factor XIII to date. From these results, it is suggested that subunit A is unstable in plasma deficient in subunit B and subunit B stabilizes the A protein. This is the first report of congenital deficiency of factor XIII subunit B and this disorder is thought to be inherited as an autosomal recessive trait.