In ten normal volunteers intramuscular injection of 0.5 mg of epinephrine caused a mean drop of 61.7% in circulating eosinophils within 3-½ hours. This epinephrine-induced eosinopenia was completely blocked by oral pretreatment with 40 mg of propranolol hydrochloride. The eosinopenic action of epinephrine is apparently mediated by a beta adrenergic mechanism. Beta adrenergic blockade with propranolol raised the blood eosinophil count of ten normal persons by a mean of 27.8%. In individual subjects the extent of this increase was inversely proportional to the control eosinophil count and directly proportional to the reduction in resting heart rate. The level of circulating epinephrine may be one of the factors which normally determine the number of peripheral eosinophils. Propranolol-induced increases in the eosinophil count may serve as an index of beta adrenergic activity.