Although it is recognized that a number of factors are involved in the mechanism controlling red blood cell production, the anterior pituitary furnishes one factor of importance. This pituitary factor is apparently a hormone distinct from other trophic hormones. It is allied chemically only to ACTH and yet both biological and chemical evidence indicates that the pituitary erythropoietic hormone and ACTH are not the same. The major problem in further elucidating the role of the pituitary in the control of erythropoiesis resolves itself into developing a satisfactory method of preparation of erythropoietic hormone which will separate it from ACTH. It is true that no ACTH preparation has been made which does not possess some erythropoietic activity, and no pituitary erythropoietic hormone has been prepared which is entirely devoid of ACTH. Nevertheless, the proportion of these two activities in different preparations has varied so greatly as to assure their separate existence. Autolytic digestion consistently reduces the ACTH activity, while erythropoietic activity is retained. It has been found that oxycellulose is a better adsorbent of ACTH than of erythropoietic hormone. These observations add chemical evidence to the accumulated biological evidence establishing the separate existence of these two pituitary principles.