In order to compare the in vitro killing activity of meropenem and imipenem against multiresistant P.aeruginosa 14 strains were used. All nosocomial isolates were susceptible to meropenem and imipenem minimum inhibitory concentration (MIC < or = 4 micrograms/ml) and resistant to at least two other antimicrobial agents of diverse chemical class with antipseudomonal activity. Forty-two killing curves were performed by exposing a 5 x 10(5) CFU/ml log-phase inoculum to 1x minimum bactericidal concentration (MBC) of each carbapenem. Meropenem was found to possess a slower killing rate than imipenem over the first 5 hours of P.aeruginosa exposure, but to be equally effective as imipenem after 24 hours of incubation. Forty percent and 11.1% of P.aeruginosa strains developed resistance to imipenem and meropenem respectively after a 24-hour exposure to carbapenem. The authors speculate about the underlying mechanisms explaining the higher rate of resistance development to imipenem than to meropenem.