To evaluate the possibility of an enhanced killing effect by ceftazidime, meropenem, or imipenem with amikacin 26 multiresistant Pseudomonas aeruginosa isolates, to nine anti-pseudomonal antimicrobials of diverse chemical classes were studied. A modified time-kill curve procedure was used with a 16 micrograms/ml concentration of each antimicrobial, i.e. within the range of their serum level; a total of 248 killing-curves were performed. Any > or = 2 log10 decrease of viable cell counts by a combination compared to the most active single agent was considered an adequate enhanced killing effect. The latter was found to be mainly expressed at 24 h of growth and involved 30-50% of the tested isolates. The above findings were independent of the MIC level to any individual beta-lactam or to amikacin. It is concluded that there is no difference between the activity of the ceftazidime and amikacin combination and those of meropenem or imipenem with amikacin on multiresistant P. aeruginosa.