Peroxidase activity in thyroid tissue from 25 patients with Graves' disease was measured by Mini assay method (J. Biochem. 98, 637-647, 1985) employing guaiacol or iodide as a second substrate. The mean values of protein-based specific activity were 0.496 guaiacol unit/mg protein and 0.187 iodide unit/mg protein, reaching 16 fold and 28 fold those of normal thyroids, respectively. The mean value of ratio of iodide unit to guaiacol unit in each thyroid, 0.68, was also much higher than that of normal human thyroid, 0.16. No significant difference in peroxidase activity was observed between patients treated with methylmercaptoimidazole and those with propylthiouracil, but the activities of those groups were significantly higher than those of patients treated with potassium iodide, suggesting that inorganic iodine therapy plays some role in suppressing the synthesis of thyroid peroxidase in vivo.