SummaryA patient with actinomycosis was found to have immediate hypersensitivity to penicillin by previously described skin testing methods as well as by a recently described in vitro system utilizing rat peritoneal mast cells. Desensitization to penicillin became imperative, but in using the classical route (the intracutaneous route), systemic anaphylaxis and other complications were encountered. Subsequently, successful desensitization was accomplished utilizing the intravenous route. We are able to demonstrate this patient's loss of reactivity to penicillin by the rat mast cell system as well as skin tests and to correlate the immunological state with the clinical course.