The in-vitro bactericidal and bacteriolytic activities and the effect on bacterial morphology of carumonam, a new N-sulfonated monocyclic beta-lactam antibiotic, against aerobic Gram-negative bacilli were compared with those of aztreonam. Both antibiotics, at their minimal inhibitory concentration (MIC) levels, were strongly bactericidal against species of Enterobacteriaceae, but weakly bactericidal against Pseudomonas aeruginosa. Their bactericidal activity was not increased by increasing the antibiotic concentration above the MICs. In contrast to this potent bactericidal activity, the bacteriolytic activity of carumonam and aztreonam was low and limited to enteric bacterial species like Escherichia coli. Filamentation was the major morphological change of Gram-negative bacilli after exposure to carumonam and aztreonam. E. coli and Serratia marcescens, but not P. aeruginosa, were converted to ghosts after prolonged incubation with carumonam.