The in vitro and in vivo antibacterial activities of carumonam (AMA-1080), a synthetic sulfazecin derivative, were compared with those of aztreonam, cefoperazone, ceftazidime, and cefsulodin. Carumonam was highly active in vitro against members of the family Enterobacteriaceae, Pseudomonas aeruginosa, and Haemophilus influenzae and weakly active against Streptococcus pneumoniae, but it was not active against Staphylococcus aureus. The MICs of carumonam for 90% of 1,156 clinical Enterobacteriaceae isolates were between 0.013 and 25 micrograms/ml, which were the lowest MICs of the antibiotics tested. The MIC of carumonam for 90% of Klebsiella oxytoca was 0.2 micrograms/ml, whereas that of aztreonam was 50 micrograms/ml. The superiority of carumonam to aztreonam and the reference cephalosporins was also demonstrated by their activities against Klebsiella pneumoniae and Enterobacter cloacae. The MIC of carumonam for 90% of P. aeruginosa was 12.5 micrograms/ml, which was comparable to the MICs of aztreonam and ceftazidime. Carumonam showed a high affinity for the penicillin-binding protein 3 of gram-negative bacteria, but not for the penicillin-binding proteins of S. aureus and Bacteroides fragilis. Carumonam was resistant to hydrolysis by 12 plasmid-mediated beta-lactamases and 7 chromosomal beta-lactamases. It was more stable than aztreonam to hydrolysis by the beta-lactamase of K. oxytoca; this stability is related to the superiority of the in vitro and in vivo activities of carumonam to those of aztreonam against this species. In general, the protective activities (50% effective dose) of carumonam and reference antibiotics in mice with experimental intraperitoneal infections correlated with the in vitro activities (MIC); carumonam showed excellent protective activity against most aerobic gram-negative bacteria.