A woman with a preliminary diagnosis of afibrinogenaemia was later found to have a functional fibrinogen of 0.06 mg/ml and markedly prolonged thrombin and reptilase times. The stoichiometry of fibrinopeptide release was normal but there was a gross delay in the polymerization of purified fibrin. Plasma protein electrophoresis showed an absence of normal fibrinogen and a novel anodal component which was confirmed as fibrinogen by immunofixation. Western blots of non‐reducing SDS‐PAGE gels indicated a molecular weight of 270 kD, compared to 340 kD for normal fibrinogen and similar analysis of reducing gels showed that the expected 67 kD Aα chain was missing and replaced by a 30 kD band. This aberrant chain was not detected by the monoclonal antibody F‐103, which recognizes the epitope formed by residues 259–276 of the Aα chain. Cycle sequencing of the DNA encoding the F‐103 epitope revealed the homozygous insertion of cytosine at position 4133 of the gene sequence. Predictably this translates as three new amino acids (²⁶⁸Gln‐Glu‐Pro) before termination at a new (TAG) stop codon. No abnormal Aα chains could be detected in plasma from the woman's heterozygous son. The hypofibrinogenaemia observed is likely to be the result of diminished assembly and/or secretion of the truncated Aα chains rather than enhanced extracellular degradation.