The animal inhibitory concentrations of 32 antimicrobial agents for 322 strains of non-fermentative gram-negative bacilli, representing 30 species and unnamed groups other than Pseudomonas aeruginosa, were determined in cation-supplemented Mueller-Hinton broth with use of a microdilution method. The activities of beta-lactam antibiotics varied, but azlocillin, mezlocillin, piperacillin, cefotaxime, and moxalactam consistently were more active than were available penicillins and cephalosporins; the penicillin-like antibiotic mecillinam was less active. The activities of the aminoglycoside antibiotics were similar to each other although streptomycin and sometimes kanamycin were less active than the other aminoglycosides. Although minocycline and doxycycline usually were more active than tetracycline, the percentages of strains susceptible to the first two drugs at concentrations achievable in vivo were only occasionally higher than the percentage of strains susceptible to tetracycline. The activity of trimethoprim-sulfamethoxazole was variable. Chloramphenicol, erythromycin, clindamycin, and vancomycin were relatively inactive against most of the organisms tested, but the high degrees of activity of these drugs against specific species or strains, particularly those that were resistant to many drugs, indicated potential clinical usefulness of these agents.