A specific double antibody radioimmunoassay for the measurement of Thyroglobulin (HTg) in human serum was developed according to the method of Van Herle. The sensitivity of the assay is 2.5 ng/ml. The mean concentration of HTg in 47 normal subjects was 15,1 ng/ml. Elevated HTg levels were found in patients with Graves' disease (12 cases) and in patients with metastases of differentiated thyroid carcinomas (14 cases). In patients with metastases of differentiated thyroid carcinoma the Tg level increases under stimulation by endogenous TSH. Sera obtained after total thyroid ablation for thyroid carcinoma did not contain demonstrable amounts of HTg. This technique was also applied to the assay of HTg in thyroid tissue.