Congenital hypofibrinogenemia is a rare disorder caused by a number of different mutations in the fibrinogen genes. The aim of the study was the elucidation of molecular defects in two unrelated families with hypofibrinogenemia. DNA samples from the patients were screened for mutations in the fibrinogen genes by direct sequencing of polymerase chain reaction-amplified gene segments. Isolated plasma fibrinogen was studied by sodium dodecyl sulfate electrophoresis and electrospray ionization mass spectrometry in order to detect variant polypeptides. Fibrin polymerization was analyzed both in plasma and using purified fibrinogen samples. A novel mutation in the FGG gene (G7590A) was found in all patients from the two families with hypofibrinogenemia. This mutation causes the amino acid exchange 313 Ser-->Asn in the gamma chain. When plasma fibrinogen from a heterozygous individual was analyzed for the presence of variant gamma chains by reverse-phase high-performance liquid chromatography and electrospray ionization mass spectrometry, only normal gamma chains could be detected. The molecular defect affecting an evolutionary highly conserved amino acid residue in human fibrinogen interferes with plasma expression of the variant molecules and is causative for the observed hypofibrinogenemic phenotype.