The cross-reactivity between the monobactam antibiotic aztreonam and the commonly used beta-lactam antibiotics, penicillins, and cephalosporins was investigated. Antibodies to aztreonam, penicillin, and cephalothin were raised in rabbits. The ability of the homologous or heterologous drug or drug conjugates to inhibit antibody binding was assessed in a solid-phase radioimmunoassay. Aztreonam demonstrated very little ability to interact with anti-penicillin or anti-cephalothin antibodies as it required 10,000-fold higher concentrations than other beta-lactams to achieve equivalent blocking. Similarly, penicillin and cephalothin conjugates did not cross-react, to any significant degree, with anti-aztreonam rabbit antiserums. Interestingly, free aztreonam was as effective as conjugated aztreonam in reacting with antibodies raised against conjugated aztreonam. This result suggested that, in contrast to the other beta-lactams, antibodies to aztreonam recognize the side chain rather than the nuclear structures. Studies with other beta-lactam analogs confirmed that the IgG rabbit anti-aztreonam binding was indeed side chain-specific. Thirty-six volunteers were given a seven-day course of therapeutic doses of aztreonam and in none did any detectable IgE anti-aztreonam antibodies develop. Four of these subjects had evidence of preexisting IgG antibodies cross-reactive with aztreonam, but the levels rose in only one patient following drug exposure. This human IgG anti-aztreonam was also directed to the side chain and did not cross-react with cephalothin or penicillin. The ability of aztreonam to cross-react with human IgE to various penicillin determinants was also investigated. Aztreonam determinants analogous to the penicillin determinants (penicillin, penicilloyl, and penicilloate) were constructed and the maximal concentration that did not evoke false-positive skin test results was determined to be 6 X 10(-3) mol/liter. None of 41 patients with documented IgE-reactive skin tests to various penicillin determinants concurrently demonstrated reproducible reactivity to any aztreonam reagents. IgE anti-penicilloyl antibodies from three persons were also tested in vitro for their ability to cross-react with conjugated or free aztreonam. Minimal, if any, reactivity was observed between the IgE anti-penicilloyl and any of the aztreonam materials. These results indicate that there is very little cross-reactivity between the monobactam aztreonam and other beta-lactam antibiotics.