The thyrotropin receptor (TSHR) and alphal-antytripsin contain a fragment of sequence composed of 6 amino acids in which 5 residues are identical. Previously, we have suggested that this region of similarity [residues 34-39: (EEDFRV) in TSHR] is not the target for Graves' disease patients' autoantibodies. To verify this suggestion, we studied the reaction of patients' sera with alphal-antitrypsin. Two methods were used: TRAK assay, normally designed to estimate anti-TSHR autoantibodies in patients' sera, and immunoblotting. A modified version of the former assay was also used to study the influence of the synthetic peptide, corresponding to the region of similarity in TSHR, on Graves' patients sera or on thyrotropin (TSH) binding, and to study the influence of this peptide antiserum on TSH binding to the receptor. The TSHR stimulating and blocking activity of antisera to this peptide was studied in transfected Chinese hamster ovary cells. No influence of alphal-antitrypsin on the binding of patients' antibodies to the receptor were noticed nor were there reactions of autoantibodies with alphal-antitrypsin. We found that patients with anti-TSHR autoantibodies had a normal concentration of alphal-antitrypsin. A peptide corresponding to residues 34-39 did not influence Graves' patients sera and TSH binding and antiserum to this peptide did not influence TSH binding and adenylate cyclase activity. In summary, the results indicated that the sequence EEDFRV is not the target for patients autoantibodies.