Antibacterial activities of amikacin, gentamicin, dideoxykanamycin B and kanamycin against clinically isolated gram-negative bacteria, 168 strains of Escherichia coli, 27 strains of Klebsiella pneumoniae, 22 strains of Citrobacter, 25 strains of Enterobacter, 20 strains of Proteus and 84 strains of Pseudomonas aeruginosa were examined by the twofold agar-dilution method and compared. The experiments revealed the poor sus ceptibility of the bacteria to kanamycin, moderate to dideoxykanamycin B and gentamicin, and excellent to amikacin. For example, all of the kanamycin-resistant strains of Klebsiella and Citrobacter were susceptible to amikacin, and 82.3% of Pseudomonas aeruginosa which were completely resistant to kanamycin were susceptible to amikacin. A number of susceptible profiles were obtained when the kanamycin resistant strains of the examined bacteria were tested against gentamicin, dideoxykanamycin B, tobramycin, amikacin, lividomycin and neomycin. From the profiles, distribution of the possible resistance mechanisms to the aminoglycosides among the gram-negative bacteria were proposed.