The pharmacokinetic properties of amikacin (BBK8) were similar to those of kanamycin in newborn infants. Peak serum concentrations of both drugs were in the range of 15 to 25 mug/ml with the exception of kanamycin in babies weighing greater than 2,000 g at birth where peak levels were 12.5 to 15 mug/ml. Volumes of distribution, plasma clearances, and serum half-life values were comparable for the two drugs. The clinical and bacteriological responses to amikacin therapy were assessed in 45 neonates with bacterial diseases. A case fatality rate of 26% was observed in infants with septicemia and/or meningitis, whereas no deaths occurred among 22 infants with urinary tract and mucocutaneous infections. Cultures from infected sites were sterile within 72 h of initiating amikacin therapy in 47% of the infants, continued positive for greater than 72 h in 31%, and were not reevaluated during therapy in 22%. The clinical response was judged to be satisfactory in 92% of the surviving infants. The efficacy of amikacin was comparable to that of kanamycin or gentamicin in neonatal bacterial diseases.