RCT 4 Randomized double-blind, placebo-controlled study of 3 groups. Neonates (n=94) fulfilling the following criteria: gestational age at birth between 32nd and 36th weeks, weight over 1500 g and no congenital defects of gastrointestinal system or other defects that prevent enteral nutrition, have been randomized to receive either placebo (microcrystalline cellulose) or a probiotic preparation (Lactobacillus rhamnosus GG, ATCC 53103) or a prebiotic preparation (a mixture of Polydextrose and Galacto-oligosaccharideOS in a 1:1 ratio). The treatment continues for 2 months.