This study documents an important change in the kanamycin in susceptibilites of Escherichia coli strains cultivated from neonates. Whereas some nurseries have in the past experienced resistance rates as high as 70 per cent recent survelliance of seven North American nurseries demonstrated that 90 percent or more of E. coli strains are currently susceptible to kanamycin. Pharmacokinetic studies of 65 babies treated with either 7.5 or 10 mg/kg kanamycin doses revealed that peak serum values varied with dosage, birthweight, and chronologic age. Peak serum levels were below the desired therapeutic range in many babies treated with 7.5 mg/kg doses of kanamycin every 12 hours. Concentrations in cerebrospinal fluid specimens from 21 infants were 0.5 to 12 mu/ml after 7.5 mg/kg kanamycin doses. Calculated distribution volumes, plasma clearances, and serum half-life values were used in formulating a revised kanamycin in dosage and frequency of administration schedule.