An inherited dysfunctional fibrinogen variant, denoted as fibrinogen Milano III, was found in a 13-year-old girl suffering from recurrent venous thrombosis. Plasma of the patient exhibited prolonged thrombin time and Reptilase time. Polymerization of fibrin monomers in the presence and absence of calcium ions was strongly impaired. SDS-polyacrylamide gel electrophoresis of reduced fibrinogen showed normal B beta- and gamma-chains, whereas no normal A alpha-chain was detected in the proposita. Immunoblot analysis with the monoclonal antibody Y18, detecting an epitope within the stretch of amino acids A alpha 1-51, revealed an A alpha-chain of about 50 kDa with an intact amino terminus. Immunoblotting with antibodies directed against serum albumin demonstrated the presence of albumin covalently linked to fibrinogen via a disulfide bridge. The structural defect of fibrinogen Milano III was determined by sequence analysis of a single-stranded fragment of genomic DNA amplified by polymerase chain reaction. An insertion of a thymine in the exon V of the A alpha-chain gene after the triplet ATT coding for IleA alpha 451 altered the reading frame and caused premature termination of the protein synthesis (Trp452(TGG)-Ser453(TCC)-Stop454(TGA)). In both parents, normal and mutant alleles were established, leading to duplication of the sequence pattern after the thymine insertion site, whereas the proposita is homozygous for the new mutation in the fibrinogen A alpha-chain gene.