Amoxycillin is currently the most widely used antibiotic in pediatrics. It is known that this antibiotic, as well as other kinds of antimicrobial treatments, may produce variations in the bowel ecosystem. A controlled randomised clinical trial was performed using commercially available fermented milk products in infants who had received an oral treatment with amoxycillin for an average of ten days. 40 children have been examined, randomly divided into two groups of twenty. Group 1 received treatment with antibiotic plus lactobacilli. Group 2 received only amoxycillin. At the end of the trial infants of group 1 showed a lower frequency of stool passages and more fully-formed feces, while patients of group 2 presented an increased frequency of gastrointestinal disorders. On the basis of the results obtained it is possible to conclude that oral bacterial therapy with lactobacilli is efficacious in preventing diarrhea from amoxycillin in infancy.