Skin testing for penicillin allergy with penicillin G (Pen G), penicilloic acid (PA), and penicilloyl poly-L-lysine (PPL) was performed on 740 subjects, and the results were assessed from epidemiologic and immunologic perspectives. Approximately 95% of these patients had histories of apparent allergic reactions to beta-lactam antibiotics, and 63% were skin-test positive. The prevalence of positive skin tests was related to the time that had elapsed between clinical reactions and skin testing. Ninety-three percent were skin-test positive 7 to 12 mo after reactions, and 22% were positive 10 yr or more after reactions. Patients under 30 yr of age had a prevalence of positive skin tests 1.7-fold higher than older patients. Testing with PPL, PA, and Pen G detected 76.3%, 55.3%, and 57.1% of the positive patients, respectively. Omission of PPL, PA, or Pen G would have led to a failure to detect 25.6%, 7.2%, and 6.2% of the positive patients, respectively. Subjects with skin tests positive to penicillin often reacted to skin tests with other beta-lactam antibiotics; 73% (41 of 56) reacted to ampicillin and 51% (38 of 74) reacted to cephalothin. No serious allergic reactions were provoked by testing. None of the 83 skin test--negative patients treated with beta-lactam antibiotics immediately after testing experienced acute allergic reactions. Two patients developed mild urticaria beginning 3 and 5 days into therapy. One skin test--negative patient experienced urticaria 3 hr after receiving oral penicillin 6 mo after skin testing. This patient's skin-test status immediately before therapy was unknown. These results support the position that testing with PPL, PA, and Pen G is a rapid, safe, and effective method for identifying patients at risk, or not at risk, for allergic reactions to penicillin.