Thyroid tissue obtained from 12 patients with Graves' disease and treated with thionamide drugs for 3-7 mo before subtotal thyroidectomy, from 12 patients with Graves' disease, similarly treated, and given 50 mug of triiodothyronine (T3) for 10 days before surgery, and from 12 euthyroid patients with solitary cold nodules was investigated to compare in vitro iodination of thyroglobulin in toxic diffuse goiter and in normal thyroid tissue. The supernates of the homogenates (105,000g) were subjected to sucrose density gradient centrifugation (5--28%) to separate the thyroglobulin fraction. The precipitates were treated with 1% digitonin and centrifuged to collect the supernate (particulate fraction). When thyroglobulin and particulate fractions obtained from the same patient were incubated with 125I-, iodide, glucose, and glucose oxidase, the amount of iodine bound to thyroglobulin was several times greater in toxic diffuse goiter than in normal thyroid tissue; administration of T3 did not affect iodination in toxic diffuse goiter. When the thyroglobulin fraction from each patient was incubated with a standardized quantity of peroxidase instead of the individual particulate fraction, the amount of iodine bound to thyroglobulin was the same among the three groups of patients. Finally, when bovine serum albumin was substituted for thyroglobulin from each of the patients, iodination of bovine serum albumin was several times greater with the particulate fraction obtained from toxic diffuse goiter tissue than with that obtained from normal tissue. The guaiacol-oxidizing activity oty. These results suggest that in vitro iodination of thyroglobulin is increased in toxic diffuse goiter even when patients are made euthyroid by treatment with thionamide drugs as well as when they are given additional T3 for 10 days before operation. The increase in iodination of thyroglobulin appears to be due to an increase in peroxidase activity in the particulate fraction.