Allergic reactions to beta-lactam antibiotics are well described; however, only the immunochemistry of penicillin has been characterised. Although the major determinant of benzylpenicillin (penicillin G) is commercially licensed for diagnostic applications, minor determinants are only available at some centres to be used for identification of hypersensitive individuals. Cephalosporins share a common bicyclic nuclear structure with penicillin and most cross-reactivity is generated to the beta-lactam ring. It is not possible to determine the actual incidence of cross-reactivity between penicillins and cephalosporins, but evidence suggests the true rate is lower than usually appreciated. In vitro testing demonstrates extensive cross-reactivity between penicillin and the carbapenems. Although the rate of clinical cross-reactivity is not known, imipenem should not be given to patients with proven penicillin hypersensitivity. In contrast, aztreonam and the monobactams can be safely given to penicillin-allergic patients.