The susceptibility of current clinical isolates of Pseudomonas aeruginosa and Enterobacteriaceae to amikacin and other aminoglycosides was tested by a standardized disk sensitivity method. Minimal inhibitory concentrations (MICs) were determined for all 200 isolates tested, and mean MICs were calculated for each of 10 bacterial species. Amikacin proved to be the most effective of six aminoglycosides against nine bacterial species; isolates of Proteus morganii were slightly more sensitive to gentamicin than to amikacin. Whereas 50% of the 200 isolates could be considered resistant to gentamicin (MIC, greater than 16 mug/ml), 94.4% of the 126 enteric gram-negative bacilli and all 74 isolates of P. aeruginosa were sensitive to amikacin. At a concentration of 8 mug/ml, gentamicin inhibited 50% and tobramycin inhibited 67% of the 200 isolates. At 16 mug/ml, amikacin inhibited 96.5% of the 200 isolates; the respective figures for kanamycin, aminosidine, and streptomycin were 28.5%, 26.5%, and 24%. The virtual absence of cross-resistance between amikacin and gentamicin and between amikacin and the other four aminoglycosides was confirmed.