A total of 434 clinical aerobic gram-negative bacillary isolates were tested against LY127935, a new 1-oxa cephalosporin, and compared with other cephalosporins, penicillins, and aminoglycosides by a broth microdilution technique. Cefotaxime (HR756), a new semisynthetic cephalosporin, and LY127935 were more active, and showed lower minimum inhibitory concentrations (ranges ≤0.12 to 2.0 μg/ml), than cefamandole, cefoxitin, and cefazolin against Escherichia coli, Klebsiella spp., Enterobacter spp., Proteus mirabilis, indole-positive Proteus spp., Serratia marcescens, Providencia spp., and Citrobacter spp. Against P. aeruginosa, piperacillin, azlocillin, and mezlocillin were the most active beta-lactam agents; 64 μg/ml inhibited 99, 93, and 87% of the isolates, respectively. LY127935 and cefotaxime at 16 μg/ml inhibited 71% of Pseudomonas isolates whereas the aminoglycosides gentamicin, tobramycin, and amikacin at a concentration of 4 μg/ml inhibited 84, 88, and 93% respectively. Minimum bactericidal concentrations were determined for all isolates and were generally the same as the minimum inhibitory concentrations.