Thyroglobulin levels were measured, without interruption of hormone treatment, in 115 patients who had undergone thyroidectomy for differentiated carcinoma. Thyroglobulin levels were invariably high in patients with iodine fixing metastases. The estimation of thyroglobulin was found to be more sensitive than the detection of metastases by their uptake of 131 iodine. A new protocol is suggested for monitoring treated differentiated carcinoma.