Penicillin erroneously dispensed as ascorbic acid caused recurrent anaphylaxis in a patient with a history of penicillin allergy. Diagnosis was delayed by the failure of the patient to admit use of the medication despite persistent questioning about drug ingestion by several examiners. The possibilities and implications of drug misidentification are discussed. In allergic reactions of inapparent cause, all medications, however innocuous their presumed identity might seem, should be evaluated.