We identified the mutations causing factor XIII A subunit deficiency in two families. Two distinct mutations were identified in the S family: the nonsense mutation Tyr 441-->stop in exon 11, inherited through the paternal line, and the missense mutation Asn 60-->Lys in exon 3, inherited through the maternal line. Two members of the J family were heterozygous for the previously described type 3 A subunit. The substitution giving rise to the type 3 variant was found to be Gly 501-->Arg in exon 12. The Asn 60-->Lys and Gly 501-->Arg mutations were constructed in cDNA clones and expressed in yeast (Saccharomyces cerevisiae AH22). Although mRNA could be detected, protein containing the Asn 60-->Lys substitution could not be detected, suggesting extreme instability or susceptibility to proteolysis. A subunits containing the Gly 501-->Arg substitution were expressed and found to be enzymatically active in fresh yeast lysates. This variant has thermal instability and lost activity during storage or purification. Gel filtration studies suggested that the type 3 variant assembled as a dimer, as do normal A subunits. The data suggest that the Gly 501-->Arg (Type 3 variant) would cause severe factor XIII deficiency if inherited in the homozygous form or as a compound heterozygote with another deleterious mutation.