BACKGROUND: Breast-fed infants, unlike bottle-fed babies, have a microbic intestinal flora characterised by a marked predominance of bifidobacteria and lactic acid bacteria. This is essentially due to the prebiotic effect of oligosaccharides in human milk. Recently, oligosaccharides with a prebiotic effect have been added to formulas. Aim. To characterise the mixture of oligosaccharides contained in these new formulas. MATERIALS AND METHODS: The characterisation of oligosaccharides was performed using thin layer chromatography as well as high performance anion exchange chromatography. RESULTS: The mixture of oligosaccharides used in the formulas analysed was made up of oligosaccharides with low molecular weight (transgalactosylated oligosaccharides) and polysaccharides with high molecular weight (inulin). CONCLUSION: With the methods employed, it was possible to characterise the mixture of oligosaccharides used as prebiotics in the formulas now available on the market.