Experience with gnotobiotes emphasizes that intestinal microflora interferes with nutritional processes in different respects (Table II). The microflora can influence the absorption of nutrients by direct action on the gut wall or the gut function or by transformation of a variety of exogenous and endogenous substances into newly absorbable or unabsorbable forms. It can influence the structure and function of a nutrient or a compound contained in food or feed, and render a nontoxic substance into a toxic one andvice versa. One has to be cautions to apply the results obtained with gnotobiotic animals to human nutrition because of morphological, physiological or microecological differences.