SummaryAfter a short review of the history of antihormones, the antigonadotrophic and antithyrotrophic hormones have been discussed in detail because these are the best known members of this group. The possible existence of other antihormones, especially of antigrowth hormone, antiprolactin, antiketogenic, anticortin, anti‐thyroid hormone, etc. has also been considered.In view of the fact that many of the relevant publications have appeared in foreign languages and in journals not readily accessible to the general public, we tried to make this review as complete as possible. We felt that in the present state of our knowledge, an unbiased presentation of all the available data which may throw light on the theoretical interpretation and practical evaluation of the antihormones would prove of more value to the reader than an elaborate discussion of theories and personal views, most of which would after all have to be based on impressions rather than on knowledge.The most important facts which, as our survey shows, are now definitely established are the following:1. The blood of animals chronically treated with gonadotrophic or thyrotrophic preparations acquires the ability to neutralize the action of such preparations in other animals.2. The antihormones are specific in so far as, for instance, antigonadotrophic hormone does not antagonize the action of the thyrotrophic hormone and vice versa.3. The blood of an animal rendered immune to a certain type of a gonadotrophic preparation, though very active in neutralizing the action of this preparation, may be relatively inactive with regard to other gonadotrophic extracts. Similar examples of extract specificity have been observed concerning the antithyrotrophic hormone.4. Neither the presence of the effector organ (the gonads in the case of the gonadotrophic and the thyroid in the case of the thyrotrophic hormone) nor that of the hypophysis is essential for antihormone formation.5. There is little or no evidence that chemically pure hormone preparations such as the oestrogens, adrenaline, etc., may form antihormones in the true sense of the word, although the sensitivity of the organism to such preparations may decrease after prolonged treatment, In summarizing the essence of this survey, the authors emphasize, however, that the fact thatpretreatment with certain hormones will cause the appearance in the blood of principles antagonistic to these hormonesmay be regarded as established beyond doubt. It is felt that the much‐debated question, whether these principles are hormones or antibodies, cannot be answered with certainty as yet; and even if we knew more about them the answer would largely depend on our definition of these two types of active princip