In the thymus extract, active in accelerating growth and development in the offspring of rats, the iodine-reducing material was analyzed. Ascorbic acid, glutathione, and cysteine, approximately account for all the iodine reduction. In fresh extracts their respective % of iodine reduction is about 55, 37, 7; their respective concs. is about 30, 65. 5 mg.%. Ergothioneine is absent.