A congenital hypodysfibrinogenemia, fibrinogen Baltimore II, was found in a young asymptomatic Caucasian female. Prothrombin, partial thromboplastin, and euglobulin lysis times were normal, as were platelet function and coagulation factor assays. Subnormal plasma fibrinogen levels were found using chronometric, rate-independent, and immunologic assay methods. Kinetic analysis of fibrinopeptide release revealed a delay in the thrombin-catalyzed release of fibrinopeptide B from the abnormal protein. Proteolysis of fibrinopeptide A by thrombin or Arvin, fibrin monomer polymerization, and fibrin polymer ligation occurred at normal rates. Catabolism of radiolabeled autologous and homologous fibrinogen was also normal, but the fibrinogen synthetic rate was less than half the normal value. Comparison of the coagulation characteristics of fibrinogen Baltimore II with those of other abnormal fibrinogens indicates that it represents a unique example of hypodysfibrinogenemia.