Ampicillin and gentamicin were given continuously i.v. to 36 newborn infants using a dosage schedule and the results were compared with those obtained in an earlier study including 88 infants who received individually calculated dosages. With the dosage schedule the variation in the serum concentrations of antibiotics was smaller in the same child throughout the treatment course, but greater between the infants. The 95% limits for the serum concentrations of antibiotics were, however, nearly the same in the two treatment groups, and the use of a dosage schedule is therefore recommended. Serum gentamicin concentration should be assayed about 3 half lives (18 hours) after beginning the treatment, and dosage adjustment be made if the serum gentamicin concentration is outside 3-5 micrograms/ml.