The coupling activity of thyroid peroxidase (TPO) in thyroid glands from patients with benign adenoma, papillary carcinoma, and diffuse goiter (Graves' disease) was measured for the first time, in addition to the peroxidase activity of these tissues. The peroxidase activity of TPO in the mitochondria-microsomes fraction was measured with guaiacol or iodide as the second substrate. In the case of papillary carcinoma, the mean protein-based specific activity obtained by the guaiacol assay was about 1/7 of that of diffuse goiter. The iodide oxidation activity of carcinoma was very low, about 1/25 [corrected] of that in diffuse goiter and 1/70 of that in adenoma. The peroxidase activity in adenoma was almost similar in the guaiacol oxidation assay and approximately one half in the iodide oxidation assay as compared with that in diffuse goiter. There was a close correlation between the guaiacol and iodide oxidation assays in individual patients with adenoma and diffuse goiter, but not in patients with papillary carcinoma. The coupling activity of TPO was measured with thyroglobulin purified from pooled toxic diffuse goiters and chemically iodinated to contain little additional T3 and T4. The specific coupling activity of TPO in mitochondria-microsomes from carcinoma was significantly lower (about 1/5) than that of diffuse goiter, and the activity in adenoma was not significantly different (about 1/2) from that of diffuse goiter. The data of coupling activities has a close correlation with that of peroxidase activities in individual patients with adenoma but not in patients with carcinoma. Based on these findings, the qualitative abnormality of TPO and its relation to the cold 123I scintigram in thyroid tumors are discussed.