The effect of S. boulardii on experimental colitis induced by the single oral administration of 1 mg/kg of clindamycin in the Syrian hamster was studied. Oral administration of S. boulardii for 13 days, starting 3 days prior to the administration of clindamycin, significantly reduced the mortality rate and inhibited the growth of C. difficile in the caecum and colon. There was a marked improvement in the sub-mucosa and mucosa which showed major inflammatory lesions when clindamycin was administered alone.