The efficacy of Lactobacillus GG yoghurt in preventing erythromycin associated diarrhoea was studied. Sixteen healthy volunteers were given erythromycin acistrate 400 mg t.i.d for a week. The volunteers were randomly assigned into two groups taking twice daily 125 ml of either Lactobacillus GG fermented yoghurt or pasteurized regular yoghurt as placebo during the drug treatment. Subjects receiving Lactobacillus GG yoghurt with erythromycin had less diarrhoea than those taking pasteurized yoghurt. Other side effects of erythromycin, such as abdominal distress, stomach pain and flatulence, were less common in the GG yoghurt group than in the placebo yoghurt group. The subjects receiving Lactobacillus GG yoghurt were colonized with these bacteria even during erythromycin treatment as measured by faecal counts of total Lactobacillus GG. No Lactobacillus GG was found in the faecal samples of volunteers in the group taking pasteurized yoghurt.