Six patients with chronic relapsing diarrhoea caused by Clostridium difficile were treated with rectal instillation of homologous faeces (one patient) or a mixture of ten different facultatively aerobic and anaerobic bacteria diluted in sterile saline (five patients). The mixture led to a prompt loss of Cl difficile and its toxin from the stools and to bowel colonisation by Bacteroides sp, which had not been present in pre-treatment stool samples. Strains of Escherichia coli, Cl bifermentans, and Peptostreptococcus productus in the mixture inhibited the in-vitro growth of Cl difficile, which in turn inhibited the growth of Bacteroides ovatus, Bacteroides vulgatus, and Bacteroides thetaiotaomicron. The finding that Bacteroides sp had been absent during the patients' illness but was present after recovery suggests that the absence of Bacteroides sp may result in chronic relapsing Cl difficile diarrhoea, and that its presence may prevent colonisation by Cl difficile.