Bacteraemia caused by Streptococcus bovis is often associated with colorectal tumours. Also, experimental studies have been proposed that S bovis acts as a promoter of tumours. We report the case of a man with colon adenoma who had a high proportion of S bovis in his faecal flora. He was treated with a Lactobacillus casei preparation (BLP) and the effect on the faecal flora was examined. L casei reduced the proportion of S bovis (from 43% down to 9%), and the effect continued after the administration of BLP was stopped. Our data indicate that BLP can repress the excessive colonisation of S bovis.