THE antigenic properties of both purified growth hormone and adrenocorticotropic hormone (ACTH) have been the subject of relatively few investigations. Using growth hormone, Elberg and Li1 have concluded that this hormone is non-antigenic. With adrenocorticotropic hormone both Gordon2 and Chase3, using different techniques, have concluded that antibodies are formed to this preparation. In this laboratory, preliminary immunizations led us to suspect the formation of antibodies to growth hormone and, as a consequence, we have reinvestigated the problem.