Preterm birth, bacterial colonization and formula feeding predispose to necrotizing enterocolitis (NEC). Antibiotics are commonly administered to prevent sepsis in preterm infants, but it is not known whether this affects intestinal immunity and NEC resistance. We hypothesized that broad-spectrum antibiotics treatment improves NEC resistance and intestinal structure, function and immunity in neonates. Caesarean-delivered preterm pigs were fed 3 d of parenteral nutrition followed by 2 d of enteral formula. Immediately after birth they were assigned to receive either antibiotics (oral and parenteral doses of gentamycin, ampicillin and metronidazole, ANTI, n=11) or saline in the control group (CON, n=13), given twice daily. NEC-lesions and intestinal structure, function, microbiology and immunity markers were recorded. None of the ANTI but 85% of the CON pigs developed NEC lesions by d 5 (0/11 vs. 11/13, P