Unknown 2 of Taurog, a soluble iodinated component formed on incubation of sheep thyroid homogenates with I131 , is apparently an inorganic oxidation product of iodide. It can be formed in a system containing inorganic iodide, an alkaline buffer, and an inorganic oxidizing agent, and also occurred in some commercial Nal131 preparations. Unknown 2 does not seem to be a natural intermediate in the formation of thyroid hormone.