Immunological and immunofluorescent studies were carried out on plasma and platelets of 3 cases of congenital factor XIII deficiency. Repeated assays carried out using different lots of antiserum showed that 2 patients lacked in reality both subunit S and subunit A. The 3rd patient had a normal subunit S and no subunit A. A tentative classification of factor XIII deficiency in 2 groups was proposed: type I, which appears to be rare, is characterized by the lack of both factor XIII subunits S and A. Type II, which appears to be relatively common, is characterized by normal and near normal subunit S and no subunit A.