One hundred fifty-two strains of Escherichia coli, Klebsiella-Enterobacter, Pseudomonas aeruginosa, Proteus species, and Staphylococcus aureus were inhibited by 3.1 μg of tobramycin/ml in a broth-dilution method and showed zones of inhibition of 16 mm or more around a 10-μg tobramycin disc in the Kirby-Bauer method. Tobramycin was most active against S. aureus, 100% of strains being inhibited by 0.1 μg/ml. All strains of E. coli, K. pneumoniae, P. aeruginosa, and indole-positive Proteus species, and 80% of Enterobacter species were inhibited by 0.8 μg of tobramycin/ml, whereas only 48% of P. mirabilis strains were inhibited by this concentration. Tobramycin was approximately twice as active as gentamicin against S. aureus, four times as active against P. aeruginosa, slightly more active against E. coli and Enterobacter species, equally active against P. mirabilis, and slightly less active against K. pneumoniae. The minimal bactericidal concentrations of tobramycin and gentamicin were the same as or twice the minimal inhibitory concentrations for all strains except those of P. aeruginosa, against which greater concentrations of both gentamicin and tobramycin were required for bactericidal activity. Tobramycin sterilized cultures of S. aureus, E. coli, and P. aeruginosa, but the rate of bactericidal action was faster with a combination of tobramycin and carbenicillin than with either antibiotic alone in the same concentrations. Tobramycin retained potency in the presence of 200 to 600 μg of carbenicillin/ml for at least 6 hr of incubation at 37 C, but lost potency in the presence of 600 μg of carbenicillin/ml by 24 hr of incubation and in the presence of 800 μg/ml by 2 hr of incubation.