Immunological and immunofluorescent studies carried out on plasma and platelets of three cases of congenital factor XIII deficiency are reported. Two of these patients were originally thought to have normal factor XIII subunit S and no subunit A. However, repeated assays carried out using different lots of antiserum showed that in reality the patients lacked both subunit S and subunit A. The false positive finding was due to the presence of a anti-factor VIII contaminant in the antiserum originally used. The third patient had a normal subunit S and no subunit A. No factor XIII antigen was found by the indirect immunofluorescent technique in normal, factor XIII deficiency and von Willebrand's disease platelets. On the contrary, by using the non-monospecific antiserum a fluorescent pattern similar to that observed by using an anti-factor VIII antiserum, had been noted. On the basis of the data presented in this paper a tentative classification of factor XIII deficiency in two groups is proposed: Type I is characterized by the lack of both factor XIII subunit S and A. Type II is characterized by a normal subunit S and no subunit A. The need for a re-evaluation of published case of factor XIII deficiency by means of monospecific antisera is indicated.