The treatment with adrenocorticotropic hormone of guinea pigs sensitized with heat-killed tubercle bacilli caused suppression of their skin reactivity to tuberculin. Similar animals treated with saline did not show this change. Normal guinea pigs treated with adrenocorticotropic hormone showed suppression of inflammation, but not necrosis, produced by intracutaneous oil of turpentine. There was slight, but probably not significant, diminution of inflammation during saline administration. Tuberculin complement-fixing antibody titers were not altered by either adrenocorticotropic hormone or saline administration. Adrenocorticotropic hormone produced marked eosinopenia and lymphopenia in guinea pigs.