Beta-lactams are responsible for more than half of the allergic drug reactions encountered in the hospital setting. Although most such reactions are mild, the potential for acute and life-threatening reactions cannot be underestimated when considering readministration of beta-lactam agents, such as the penicillins or cephalosporins, to persons who have previously exhibited sensitivity. In addition to concerns about possible allergic reactions to the beta-lactam antibiotics individually, considerable cross-reactivity has been demonstrated among such classes as the penicillins, the cephalosporins, and the imipenems, although it cannot yet be predicted on an individual basis. Early studies of the unique monocyclic beta-lactam--or monobactam--aztreonam, indicated that the new class demonstrated negligible cross-reactivity with the standard beta-lactams both experimentally and clinically.Further, aztreonam was associated with an extremely low (2 percent) incidence of immunologic drug reactions. Aztreonam also has been found to be well-tolerated by highly penicillin-allergic patients. Although further clinical study is indicated, data so far are encouraging. If it is confirmed that monobactams such as aztreonam are minimally cross-reactive, well-tolerated by subjects allergic to other beta-lactam antibiotics, and only weakly immunogenic, fewer allergic reactions may be associated with antimicrobial therapy in the future than have been seen with the other available beta-lactam antibiotic drugs.