Soluble and particulate iodoproteins solubilized by detergent from in vivo <sup>125</sup>I labeled congenital goiter, have been isolated and characterized. The soluble iodoproteins (S<sub>1</sub>) were decreased but contained 80% of radioiodine; 19S thyroglobulin (TG) represented only 9% of the soluble extract, the main component being iodinated serum albumin. The solubilized iodoproteins (S<sub>2</sub>) consisted of TG-like material and represented one third of the total TG, sedimented as 19S and possessed the immunoreactive properties of native TG. The iodine content of TG from S<sub>1</sub> and S<sub>2</sub> was 0.01% and 0.008%, respectively, and no radio-labeled truodothyronine or tetraiodothyronine was detectable in the two TG containing only monoiodotyrosine and diiodotyrosine. S<sub>1</sub> and S<sub>2</sub> TG showed the same resistance to dissociation at alkaline pH in low salt. The sialic acid content was markedly decreased in both S<sub>1</sub> and S<sub>2</sub> TG, the particulate protein contained 0.06%, compared to 0.12% found in the soluble goitrous fraction and to 1.3% described in the normal soluble TG. The results presented, whilst confirming the striking decrease of soluble TG and the increase in the particulate iodoproteins, lend support to the hypothesis that TG synthesis is impaired in some cases of human congenital goiter. Moreover, our findings demonstrate the presence of membrane-bound 19S molecules and a decreased sialic acid content in the goitrous TG, which is more marked in the particulate fraction. These results suggest that a defective secretion into the follicular lumen of fully synthesized TG could be responsible for the dishormonogenetic errors in some cases of congenital goiter.