Amikacin is a new aminoglycoside antibiotic pharmacologically similar to kanamycin. It has a wide range of activity against Gram-negative bacilli, including many resistant to gentamicin. Thirty-six serious Gram-negative bacillary infections were treated with amikacin. Twenty-nine patients (80-6%) responded (cured or improved). Twelve of 13 patients with gentamicin-resistant pathogens responded. Minor ototoxicity occurred in 6 patients and was associated with prolonged therapy or previous aminoglycoside therapy. Possible nephrotoxicity with amikacin was found in 6 patients. Amikacin should be used primarily to treat suspected or known gentamicin-resistant pathogens.