Seventy-nine patients with acute or former reactions to penicillin were investigated by a benzylpenicilloyl (BPO)-specific RAST and/or by skin tests with penicilloyl-polylysine (PPL), benzylpenicillin and penicilloic acid and the results were correlated with the different clinical pictures. Positive RAST and skin test results could be found in patients with anaphylactic shock, urticaria and serum sickness-like reaction and sometimes in a special group of exanthems, which are characterized by the existence of many different lesions at the same time, therefore called 'polymorphic exanthems', and often observed after high-dosage penicillin therapy. In cases of scarlatiniform or morbilliform exanthems no positive results were found. The BPO-specific RAST showed an overall correlation of 95-I% with skin tests using PPL. However, some patients with positive skin tests to benzylpenicillin and penicilloic acid did no have detectable circulating IgE antibodies to BPO. This emphasizes the need for including these antigens in in vitro methods. The RAST was informative even at the allergic reaction or in the first 15 days afterwards and seems to be very valuable for early diagnosis of penicillin allergy especially in cases when many drugs have been given.