A qualitatively abnormal fibrinogen was detected in the plasma of a 53 year old woman with severe arterial thrombotic disease. The concentrations of plasma fibrinogen and serum fibrinogen related material were normal. The abnormal fibrinogen was electrophoretically normal. The defects detected were an abnormality of polymerization of fibrin monomers and a decreased rate of release of fibrinopeptide A. The absorption of radiolabelled partially degraded plasminogen on to fibrin prepared from purified fibrinogen Copenhagen was normal.