Amikacin sulfate was used in 24 treatment courses for 25 serious infections caused by aerobic or facultative anaerobic Gram-negative organisms resistant to numerous drugs. Sites of infection included urinary tract (11 cases), pleuropulmonary (6 cases), primary bacteremia (5 cases), and miscellaneous (3 cases). Serratia marcescens and Pseudomonas sp accounted for 73% of the isolates. The mean minimal inhibitory concentration (MIC) of these organisms to amikacin was 3.6 microgram/ml; to gentamicin, 39 microgram/ml; and to tobramycin, 32 microgram/ml. The mean peak serum concentration of the drug was 20.8 microgram/ml. Eleven patients were critically ill at the onset of therapy, and seven patients were bacteremic. The overall favorable response rate was 80%. The most serious side effect was ototoxicity, which occurred in three of 15 patients examined by serial audiometry.