For the first time, a correlation between a specific fibrinogen abnormality and the clinical symptoms of thrombosis has been found in unrelated families. These abnormal fibrinogens have been designated Dusart and Chapel Hill III. The abnormal fibrinogen Chapel Hill III was identified previously in a patient with thrombotic disease. We purified fibrinogen from small aliquots of patient and normal plasmas by a simple, rapid procedure. Coomassie stained sodium dodecyl sulfate- polyacrylamide gel electrophoresis (SDS-PAGE) analysis demonstrated that fibrinogen Chapel Hill III contained several high molecular weight forms in addition to the two forms seen with normal fibrinogen. Immunoblot analysis of Chapel Hill III fibrinogen demonstrated that essentially all the high molecular weight forms react with antiserum to albumin. Immunoblot analysis of plasmin digests of Chapel Hill III fibrinogen demonstrated that albumin is linked to the C-terminus of the A alpha chain. Using DNA analysis, we found that the patient is heterozygous for a single base change, resulting in the substitution A alpha Arg 554-->Cys. This is the same change identified in fibrinogen Dusart. The Dusart family members who are heterozygous for this substitution also suffer from recurrent thrombotic disorders.