Carumonam is a monobactam with a beta-carbamyloxmethyl group at position 4. It inhibited 90% of Enterobacteriaceae at less than or equal to 8 mg/l and had in-vitro activity similar to that of cefotaxime, ceftazidime and aztreonam. Fifty per cent of Enterobacter, Citrobacter and Serratia isolates were inhibited by 4 mg/l, but isolates resistant to aztreonam and ceftazidime were not inhibited. Carumonam, like aztreonam, did not inhibit Gram-positive or anaerobic species. Carumonam was not destroyed by the common plasmid- and chromosomally-mediated beta-lactamases and was more stable than aztreonam to attack by the K-1 beta-lactamase of Klebsiella oxytoca. Carumonam inhibited Richmond-Sykes type Ia and Id beta-lactamases but was a poor inhibitor of type III enzymes. It did not induce beta-lactamases.