A new assay method for thyrotropic hormone (TSH) based on the in vitroiodination of thyroglobulin, is presented. After incubation of hog thyroid slices for 15mins. in a modified Krebs-Ringer phosphate buffer containing 0.05μgI/ml of iodide labeled with I¹³¹ and TSH, the radio-activity in the protein fraction of slices was measured and utilized as an index of TSH activity. The usable range of this assay was 1 I.mU./ml to 30 I.mU./ml of TSH and the index of precision varied from 0.15 to 0.30. Effects of temperature, pH and constituents of the medium upon the response were studied.