Five overlapping cDNA clones representing the entire mRNA for human thyroid peroxidase (TPO) have been isolated from a human Graves' thyroid cDNA library. The cDNA sequence has been determined. Human TPO cDNA contains 3060 bases from the start of transcription to the beginning of the poly (A) tail at the 3'-end. The derived amino acid sequence of human TPO consists of 933 amino acids with a mol wt of 102,937. The derived amino acid sequence contains five potential glycosylation sites (Asn-X-Ser/Thr), a probable transmembrane signal peptide sequence at the amino terminus, and a hydrophobic putative membrane-spanning region beginning 85 amino acid residues from the carboxyl terminal end. Comparison of the human TPO amino acid sequence to that of pig TPO shows strong homology extending from the amino terminus to within 44 amino acid residues of the carboxyl-terminus.