The soluble proteins from the thyroid of a 16-yr-old girl with congenital goiter, hypothyroidism without deafness and a positive KSCN discharge test have been studied. Only 1% of the labeled iodine in the thyroid homogenate was precipitable with TCA. By paper chromatography of the digested thyroid homogenate 100% of the labeled iodine was iodide. The soluble proteins were fractionated on Sephadex G-200 and studied in a Spinco analytical ultracentrifuge. The first of the 3 protein peaks precipitated with antihuman thyroglobulin rabbit serum on Ouchterlony immunodiffusion plates. It comprised 16% of the total soluble protein and contained 2 principal proteins with sedimentation constants of 17 and 11.8. The second peak, comprising 53% of the recovered protein, was hemoglobin. The third peak, containing 31% of the recovered protein, overlapped the single 131I peak. All the radioactivity was lost from this peak after dialysis for 48 hr. It was formed by a single protein with a sedimentation constant of 4. The interpretation of these findings is discussed.