SummaryPassive cutaneous anaphylaxis (PCA) was elicited with penicillin‐treated serum, but not with penicillin itself. Although the intravenous injection of penicillin before challenge prevented systemic anaphylaxis, PCA was not inhibited in this way.Penicilloyl‐polylysine and dialysed penicillin‐treated rabbit serum produced positive PCA reactions in animals sensitized with serum from a guinea‐pig which had been immunized with penicillin‐treated homologous serum. This confirmed that penicilloyl compounds are important antigenic determinants derived from p