The in vitro susceptibility of 103 well-characterized strains of Pseudomonas aeruginosa to nine antimicrobial agents was assessed by means of the Kirby-Bauer disk diffusion assay and the microtiter minimal inhibitory concentration assay. The antimicrobials, from the most to the least active against P aeruginosa, were thienamycin greater than ceftazidime greater than piperacillin greater than azlocillin greater than cefoperazone greater than aztreonam greater than ticarcillin greater than ticarcillin-clavulanic acid greater than mezlocillin. The resistance patterns of the antimicrobial agents suggest that P aeruginosa resistant to a penicillin, cephalosporin, or aztreonam may be susceptible to thienamycin.