<jats:p> Tobramycin (factor 6 of the nebramycin complex) is a new aminoglycoside antibiotic isolated from <jats:italic>Streptomyces tenebrarius</jats:italic> which is active against <jats:italic>S. aureus, Enterobacteriaceae</jats:italic> , and <jats:italic>Pseudomonas aeruginosa</jats:italic> . Susceptibility to tobramycin of 96 strains of <jats:italic>P. aeruginosa</jats:italic> , including 45 recent isolates from blood, was studied by using agar and broth dilution methods. The minimum inhibitory concentration (MIC) for 83 of 96 strains was 3.12 μg/ml or lower in Mueller Hinton agar; MIC values were two to eight times lower in Mueller Hinton broth tests. Agar dilution MIC values were generally lower than those obtained in parallel tests with gentamicin. Killing curves obtained from serial sampling of broth cultures showed a 100- to 10,000-fold decline in viability of log-phase organisms within 30 min of exposure to the drug. Two-dimensional agar dilution tests with carbenicillin and tobramycin with 79 strains showed additive or synergistic effects; no antagonism was documented. Seventy-eight of 79 strains were inhibited by a combination of 50 μg of carbenicillin per ml and 1.56 μg of tobramycin per ml, blood levels which seem attainable in man. Tobramycin appears to be a potent, rapidly bactericidal antibiotic against <jats:italic>P. aeruginosa</jats:italic> and merits clinical evaluation.