Severe constitutional reactions, ranging from dyspnea, cyanosis, and hypotension to collapse and death, sometimes follow apparently slight provocation. Most severe allergic reactions and deaths in recent years have followed the administration of penicillin. To prevent such reactions it is important to refer to the patient's medical history. In the case of penicillin, skin testing is indicated but must be done with caution; cutaneous testing should precede intracutaneous testing, and initial concentrations as low as 0.1 unit per cubic centimeter have been recommended. Reactions that occur in spite of such precautions must be treated promptly by intramuscular injection of 0.3 to 0.5 cc. of 1:1,000 aqueous epinephrine, after which an antihistamine is to be considered. The physician should also have well in mind the steps to take when allergic reactions follow the administration of aspirin to aspirin-sensitive asthmatics.