An indirect test for the detection in man of immediate type hypersensitivity to penicillin G is described. It is based on an iodonitrotetrazolium stain for succinic dehydrogenase in the basophil leucocyte granules. Rabbit basophils, enriched by differential centrifugation, and passively sensitized by exposure to plasma from subjects with known penicillin sensitivity, showed in the presence of penicillin a marked reduction in this INT formazan staining, as compared with those exposed to control plasma.