Reaginic antibodies (IgE) to penicilloyl were detected by an in vitro method, a radioimmunological technique (RAST), in the sera of nine patients out of eleven with a recent history of penicillin hypersensitivity of the immediate type. Provocation tests in two patients with negative RAST were negative as well as in twelve patients having had a possible penicillin hypersensitivity reaction 7 months to 14 years ago. Results from skin tests and RAST agreed. Skin tests and RAST reactions were negative for thirty-two patients with a history of penicillin reactions of the delayed type. The results of this study indicate that the RAST is a valuable alternative to the more dangerous intracutaneous test with penicilloyl-polylysine for-detection of penicillin hypersensitivity of the immediate type.