The products called functional foods, which besides being merely nutritional have different beneficial effects on the organism, are situated in the context of diet and health promotion. Amongst these functional foods we can distinguish, amongst others, between probiotic and prebiotic compounds. The micro-organisms most widely used in probiotic foods belong to the Lactobacillus and Bifidobacterium types. In this article we have studied the effect of diets supplemented with Lactobacillus casei or Bifidobacterium bifidum on animal development and especially on the intestinal function, centred on their immune, digestive and absorptive activity in growing animals. The bacteria strains used modify the activity of the small intestine of healthy mice, significantly affecting their enzymatic activity (sucrase, maltase and aminopeptidase) and the collection of nutrients (galactose and glycilsarcosine), as well as the intestinal immune activity (higher number of Peyer's patches). However, these effects do not appear to disturb the development of the growing animals since no significant differences are appreciated in their body weight or in their blood parameters. These results make clear the possible beneficial effects on intestinal physiology and contribute to the understanding of the possible mechanisms of action of the probiotics, which could be employed in the preventive treatment of different pathologies related to the digestive apparatus.