The distribution of iodotyrosines and iodothyroinines has been studied in poorly iodinated 19 S thyroglobulins isolated by sucrose gradient centrifugation from five human thyroid tissue. The 19 S thyroglobulins have been fractionated by isopycnic centrifugation and 34.5% rubidium chloride gradient. The distribution of iodoamino acids depends on the total iodine content of each fraction. The variations in MIT) and DIT residues with increasing levels of iodination are not identical, the elevation of DIT residues being higher than that of MIT. The number of T4 residues increases quite rapidly after 5 atoms of iodine while T3 increases slightly and slowly. It is shown here that, even after fractionation of poorly iodinated thyroglobulin, the synthesis of thyroid hormones is observed at a level of iodine content as low as 1.4 and 1.8 atoms per molecule of thyroglobulin. This shows that the thyroglobulin fractions obtained by isopycnic centrifugation are still heterogeneous, although less than the initial non-fractionated thyroglobulin. In order to explain our data, it must be admitted that a large proportion of thyroglobulin molecules are not iodinated.