Fibrinogen Baltimore I is one of the very first congenital abnormal fibrinogens reported over several decades ago; however, the molecular defect of this dysfibrinogen has eluded identification. In fact, several reports misidentified the functional defect of Baltimore I, which has impaired fibrin monomer polymerization. Reversed-phase high-performance liquid chromatography analysis of lysyl endopeptidase digest of the purified Baltimore I γ-chain showed an abnormal peptide not found in the co-existing normal γ-chain of this heterozygote. Amino acid sequencing of this peptide indicated that γ-chain Gly292 is replaced by valine. This observation was confirmed, and the genetic defect was determined by direct nucleotide sequencing of a polymerase chain reaction product containing codon γ292, which is mutated: GGC → GTC. The molecular defect of Fibrinogen Baltimore I lies in a region of the γ-chain required for fibrin polymerization, suggesting that the integrity of γGly292 is critical for fibrin assembly.