THIS paper reports the presence in pituitary extracts of a material which inhibits the milk clotting activity of crystalline pepsin. The inhibitor was found concentrated in commercial preparations of thyrotropin, with increased inhibitor activity present in preparations most potent with respect to thyrotropin activity. Recently, at this institution, a chromatographic method for the partial purification of thyrotropin was developed, and a further fractionation by counter-current distribution has also been accomplished (1, 2). Assays of samples made available to us from these fractionation procedures provide strong evidence that the pepsin inhibitor is a separate entity from material causing stimulation of P32-uptake in chick thyroids. Details of the method used for assaying pepsin inhibition are presented.