At Montefiore Hospital, in the course of the last ten years, we have had an opportunity to study the effects of administration of large quantities of radioiodine to a considerable number of patients with metastatic thyroid carcinoma. Relatively early in our experience it became evident that radioiodine dosage is limited primarily by the effects of radiation delivered to the hematopoietic system by the circulating isotope. Assuming that the radiation dose to the blood-forming organs is correlated with the radiation dose to the circulating blood, studies of blood radiation dosage during radioiodine therapy, together with the resulting effect on the hematologic picture, may establish the degree of radioisotope toxicity. Marinelli and Hill (1) studied the blood radioiodine concentration in a number of patients, including 7 of the Montefiore Hospital series, and have calculated the resulting radiation received by the blood. Rawson, Rall, and Peacock (2) have discussed the effects of irradiation on some of the blood components due to the circulating I131 and have proposed limitations on radioiodine dosage. Stanbury et al. (3) have reported blood and “plasma radiation” doses and their effects on the blood counts following the administration of large therapeutic amounts of I131. On the basis of the tissue concentration of I131 determined at necropsy, several investigators (4–6) have reported the number of equivalent roentgens delivered to various organs. The present study, a preliminary report of which has been published (7), is concerned with the radiation dose delivered to the blood during the various stages of I131 therapy for metastatic thyroid carcinoma, and with concomitant injury to the hematopoietic system. We have studied the influence of benign and malignant functioning thyroid tissue on the blood radiation dose, which has been calculated from the blood I131 concentration and the disintegration scheme of the isotope. Approximate values for the generalized total-body radiation have been estimated from the isotope concentration in blood, a readily available and measurable tissue whose radioactivity contributes a major portion to the generalized radiation. We have not considered radiation damage to specific organs due to their proximity to depots of radioiodine, such as the thyroid gland and urinary bladder. In order to clarify further the effect of thyroid function on blood radioiodine levels, several hyperthyroid patients were similarly studied during radioiodine therapy. Procedures and Calculations Patients in this study received oral doses of I131, most of which were assayed in “New York millicuries” (8). This unit, employed since 1947, is within 10 per cent of the current Bureau of Standards and Oak Ridge millicuries. Determinations were made of the blood I131 concentration following 69 therapeutic doses to 26 patients with metastatic thyroid carcinoma.