The disruption of the natural flora of the gastrointestinal tract (especially Lactobacillus acidophilus) may occur during antibiotic therapy. This may lead to diarrhea, dehydration, and electrolyte imbalances. It has been suggested that replacement of the lactobacilli with a commercially available product may prevent the diarrhea. The efficacy and safety of prophylactically administered Lactinex (culture of L. acidophilus and L. bulgaricus) was compared with placebo for the prevention of amoxicillin-induced diarrhea in pediatric patients. Lactinex or placebo was administered four times a day for ten days to coincide with the antibiotic therapy. The Lactobacillus preparation did not appear to consistently prevent diarrhea in this patient population. Patients' age, diet, and parental definition of diarrhea were factors that may have influenced the results.