DURING a study1 of the hydrolysis of native hog thyroglobulin labelled with iodine-131, by a purified protease isolated from hog thyroid glands, fractions eluted from paper chromatograms of the enzyme fission products were treated with 2N sodium hydroxide at 105° for 16 hr. Control aliquots were allowed to stand for the same time at 0°. On application of n-butanol extracts of the heated and control samples to paper and development of the chromatograms with n-butanol – n-pentanol – ammonia2 as solvent, unexpected radioactive spots of high RF (at 0.52 and 0.68) were observed with the heated samples. Since the original eluates were found to contain free mono- and di-iodo-tyrosine and thyroxine, the action of alkali on purified samples of the iodo-amino-acids was examined.