Collip and Anderson [1934] first demonstrated the antihormone properties in the serum of rats receiving prolonged courses of injections of anterior pituitary extract. Since then such inhibitory sera to many different pituitary and other hormones have been prepared in a variety of animal species. Opinion is still divided as to the nature of the inhibitory substances. Collip and his co-workers regard them as antagonistic hormones, basing their views mainly on the results of experiments which show that an animal can produce inhibitory substances to pituitary hormones of its own species [Selye, Collip, and Thompson, 1934; Collip, 1937], and believe that such inhibitory substances may be normally present in the serum. Results obtained by other investigators, however, show that continued injection of hormone extracts from a species foreign to that used for the production of antihormones is essential [Katzman, Wade, and Doisy, 1937; Thompson, 1937]. There is much in favour of