The bactericidal activity of imipenem against Pseudomonas aeruginosa, Enterobacteriaceae and Staphylococcus aureus was compared with that of other antibiotics, using the time-related kill rate curve method. The activity of imipenem was found to be less rapid and less intense than that of aminoglycosides, similar to that of fluoroquinolones and more rapid than that of beta-lactam antibiotics, notably ticarcillin and ceftazidime against P. aeruginosa and cefotaxime against Enterobacteriaceae. As with aminoglycosides, this bactericidal activity was concentration-dependent against Enterobacteriaceae and S. aureus but not against P. aeruginosa. Unlike the beta-lactam antibiotics, but like aminoglycosides and fluoroquinolones, imipenem remained active against "quiescent" P. aeruginosa, though not against Enterobacteriaceae. Successive transfers into liquid media rapidly produced resistant variants among P. aeruginosa strains and methicillin-resistant S. aureus strains, but not among Enterobacteriaceae. This shows that the bactericidal activity of imipenem is markedly different from that of other beta-lactam antibiotics.