Serum samples from 29 patients receiving therapeutic doses of 131I were fractionated by gel nitration in columns of Sephadex G-200. Thyroglobulin could be identified in circulating serum in 10 of the cases in amounts ranging from 2.0 to 14.7% of the total protein-contained 131I. Traces (less than 1 % of the protein-contained 131I) could be observed in 6 more cases. An iodinated protein containing butanol unextractable iodine and showing the physical properties of a 7S globulin could be identified in 4 cases. In all instances the bulk of the radioactivity was contained by a protein fraction with a relatively low molecular weight (4S or smaller). Individual variations in thyroid tissue sensitivity to radiation were apparent from the varying amounts of circulating 131I thyroglobulin in different patients after similar doses of 131I. Molecular sieve filtration through Sephadex G-200 seems to be an efficient method for the identification of thyroglobulin in serum and for the removal of smaller molecular weight impurities from saline extracts of thyroid tissue.