Susceptibility of 341 isolates of non-fermentative gram-negative bacteria to carbenicillin, piperacillin, cefoperazone, moxalactam, cefotaxime, ceftizoxime, and N-formimidoyl thienamycin was determined by the agar dilution and disc diffusion methods. Piperacillin was the most active agent against Pseudomonas aeruginosa, thienamycin the most active against Pseudomonas fluorescens and Pseudomonas putida, and moxalactam the most active against Pseudomonas maltophilia. Piperacillin and thienamycin were the most active agents against the other Pseudomonas species studied. Thienamycin proved to have excellent activity against Acinetobacter calcoaceticus--90% of strains were inhibited by less than or equal to 1 microgram/ml. The two most active drugs against Alcaligenes species were piperacillin and thienamycin, both of which inhibited 90% of isolates at a concentration of 2 micrograms/ml. All drugs were active against Moraxella species. The broad sensitivity spectrum of piperacillin, thienamycin, and the third-generation cephalosporins against non-fermentative gram-negative bacteria indicates their potential use in infections caused by these organisms.