Cloned cDNA templates of thyroid peroxidase (TPO) have been used in conjunction with the polymerase chain reaction (PCR) to express selected segments of the thyroid microsomal/peroxidase antigen (TMA/TPO) as recombinant protein in E. coli. Six small, different recombinant fragments averaging 120 amino acid residues and one large fragment (269 amino acids) of TPO which together encompass 80% of the extracellular region of the molecule have been produced and autoantibody (aAb) binding sites analysed by immunoblotting. A minimum of six independent, sequential antigenic determinants have been localized on the recombinant proteins and these map to the amino terminal, the central core region and the carboxyl terminal of the TPO molecule. More accurately, the six antigenic sites reside on overlapping recombinant TPO preparations termed R1a + R1b (residues 1 to 160) R1c (residues 145 to 250), R2b (residues 457 to 589), R3a (residues 577-677), R3b (residues 657-767) and R3c (residues 737-845). The large fragment of TPO termed R3 (residues 577-845) encompassing R3a, R3b and R3c also reacts with the aAbs. Different sera from patients with autoimmune thyroid disease contain antibodies to TMA/TPO which differ in their fine specificity. The use of recombinant molecular biological techniques together with PCR to prepare small segments of a large autoantigen as recombinant protein will now allow studies to progress on autoepitope mapping of the precise amino acid sequences of the TPO molecule with the use of synthetic peptides.