A human-mouse hybridoma has been produced by fusion of Hashimoto thyroid lymphocytes with the mouse myeloma line X63-Ag8.653. The cloned hybridoma secreted 2.5µg per 106cells per day of an IgG kappa thyroid peroxidase (TPO) autoantibody (2G4) with high affinity (2.5×109molar−1) and specificity for human TPO. 2G4 did not react with lactoperoxidase, horseradish peroxidase or human myeloperoxidase or with porcine TPO or with human thyroglobulin. Plastic tubes coated with 2G4 bound about 50% of125I-labelled human TPO added and the binding was inhibited by IgGs prepared from 18/18 TPO autoantibody-positive sera. This indicated that all 18 sera contained autoantibodies which recognised the same (or closely related) epitope as 2′34. Plastic tubes coated with IgGs from different TPO autoantibody-positive patient sera also bound125I-labelled TPO but inhibition by 2G4 in this system was not complete. This suggested that the sera contained at least 2 types of TPO autoantibodies. with only one type of autoantibody reactive with the same epitope as 2G4.