Both lesion (L) and adjacent normal (N) thyroid tissue from 48 patients with non-functioning adenomas and adenomatous goitres were assayed for peroxidase activity by the 'mini' assay method employing guaiacol or iodide as the second substrate. A considerable proportion of thyroids (46% of adenomas and 22% of adenomatous goitres) demonstrated no iodide oxidation activity in L although they had guaiacol oxidation activity, and these were grouped as subgroups A. The rest of these non-functioning tumours, termed subgroups B, had both guaiacol and iodide oxidation activity which was higher (3.0-4.6 times in guaiacol assay and 7.3-14.1 times in iodide assay) in L than in N. These data indicate that the non-functioning in subgroups A may be due to a lack of iodide oxidation activity and that some other defects such as an iodide transport defect may be involved in subgroups B. Furthermore, a precise and rapid assay method for thyroglobulin iodination activity of thyroid peroxidase was developed, with modifications of previous methods. On the basis of this method, we found that there is a good correlation (r = 0.94) between iodide oxidation assay and thyroglobulin iodination assay, leading to the conclusion that thyroglobulin iodination assay can be replaced by iodide oxidation assay.