• To evaluate the effects of parenteral antibiotics on carbohydrate tolerance, fecal reducing substances, stool frequency, and dietary manipulation were monitored in two groups of term neonates. The first group (n 25) consisted of neonates receiving parenteral ampicillin sodium and gentamicin sulfate. A second group (n=25) of matched neonates not receiving antibiotics served as controls. Most of the neonates were fed a standard lactose-containing formula and the remainder were breast-fed. Stool frequency, presence of fecal reducing substances, and requirement for dietary manipulation were significantly greater in neonates receiving antibiotics compared with controls. Infants manifesting symptomatic carbohydrate intolerance while receiving parenteral antibiotics may benefit either from receiving lactose-free formula or from decreasing the intake of lactose for the duration of antibiotic therapy.