RESEABCH on the general phenomenon of compensatory organ growth is usually predicated on the assumption that humoral growth-regulating factors must exist as a logical, though hypothetical, necessity. Yet their unequivocal demonstration is, unhappily, a very rare event indeed. If our knowledge of the control mechanisms which govern the growth and development of various tissues and organs is to parallel that which has followed the discovery of erythropoietin, for example, and its role in stimulating erythropoiesis, then it is imperative that the existence of tissue-specific stimulators or inhibitors be experimentally demonstrated. Until this is achieved, the validity of many theoretical interpretations must remain in question.