Reduction in absolute numbers of circulating lymphocytes and eosinophils was observed after administering 25 cc. intravenous doses of aqueous ACE to normal adults. Such changes did not occur after a comparable intravenous dose of physiologic saline. Patients with infectious mononucleosis showed hematologic responses to ACE and ACTH both qualitatively and quantitatively similar to those observed in normal persons, while a small group of persons with chronic lymphatic leukemia failed to respond with a characteristic fall in lymphocytes either after ACTH or after doses of ACE twice as great as those effective in normal persons and in patients with infectious mononucleosis. A single individual with Addison’s disease responded to 25 cc. of ACE by vein with a more prominent lymphocyte reduction than was observed in any of the normal subjects studied. No changes in heterophile antibody titers or titers of enteric agglutinins were noted in persons with infectious mononucleosis or normal subjects within eight hours after the administration of ACTH or aqueous ACE.