Ampicillin and gentamicin were dissolved once a day in an L-amino acid solution especially prepared for parenteral nutrition of newborn infants and infused continuously to 88 infants in whom septicaemia was suspected or had been proved. The mean dosages were 162 and 5.3 mg/kg per 24 hours respectively, and the 95% limits for the serum concentrations were 11-133 and 1.3-7.4 micrograms/ml. The treatment results were at least as good as with intermittent intramuscular or intravenous administration. This new mode of giving antibiotics is less painful to the babies and easier for the nurses.