SUMMARY The presence of a single methionine in porcine parathyroid hormone, at position 8, permitted assessment of the role of this residue separate from the second methionine residue found at position 18 of bovine and human parathyroid hormones. Oxidation of the solitary methionine of porcine parathyroid hormone to the sulphoxide destroyed biological activity, but this was restored by subsequent reduction with cysteine. Oxidation of the hormone did not, however, affect its immunological activity; therefore, oxidation of the hormone may bring about dissociation of biological and immunological activity.