Ampicillin and gentamicin were administered continuously intravenously to 88 newborn infants using individually calculated dosages. For infants with a mean value of plasma clearance of the antibiotics, it was calculated that the serum ampicillin and gentamicin concentrations would be between 35-55 and 3-5 micrograms/ml, respectively, using the dosages for intermittent treatment. These dosages are therefore recommended as fixed dosages for continuous intravenous infusion initiated by a bolus dosage. Serum gentamicin concentration should be assayed about three half-lives after start of infusion and the dosage adjusted for values outside 3-5 micrograms/ml.