Aztreonam is a novel antimicrobial agent belonging to the monobactam class of antibiotics. It inhibits both beta-lactamase-producing and non-beta-lactamase-producing aerobic gram-negative bacilli, but it has no activity against gram-positive species or against anaerobic species. The efficacy of aztreonam in the treatment of infection in 76 patients and its safety in 87 patients was evaluated. The majority (91 percent) of patients had significant underlying disease, and 47 percent were critically ill. Aztreonam produced an overall clinical response of 86 percent, with 10 of 11 cases of bacteremia cured, including four due to Pseudomonas aeruginosa, seven of eight cases of pneumonia, and seven of nine episodes of osteomyelitis. Infections due to bacteria resistant to ampicillin, carbenicillin, cefazolin, cefamandole, cefoxitin, and gentamicin were cured. Although 15 of 18 patients with exacerbations of pulmonary infection due to P. aeruginosa showed clinical improvement, bacteriologic cure was not achieved, as has been noted with other drugs. Similarly, patients with major underlying structural abnormalities of the urinary tract showed early relapses of bacteriuria. Aztreonam combined with antistaphylococcal, antistreptococcal, or antianaerobic agents provided an alternative to aminoglycoside use in these non-neutropenic patients. Administration of 1 or 2 g every eight hours yield serum bactericidal levels well in excess of 1:8 against all Enterobacteriaceae and some P. aeruginosa strains. There was a low incidence of adverse side effects, none serious. Overall, aztreonam is a useful alternative to the drugs available for use in hospital-acquired gram-negative infections and provides a chance for more directed therapy.