PENICILLIN HAS become a valuable model for the study of allergic drug reactions, not only because of its widespread use but also because of the detailed chemical information on its reactivity and metabolism which has been available for almost two decades.1 However, studies on the nature of the penicillin antigen only recently established the fact that certain products of penicillin are chemically more reactive than penicillin itself, and thus more likely to act in the body as sensitizing haptens. Specifically, the reaction of penicillenic acid with proteins to form an antigenic penicilloyl-protein complex has been substantially confirmed.2,3 After Ley et al4 reported that sera from certain patients treated with penicillin agglutinated human erythrocytes preincubated with that drug, a separate line of study was pursued by several investigators to determine whether an association with clinical penicillin allergy could be made. However, certain of the results and interpretations which