The effects of two levels of transgalactosylated oligosaccharide (TOS) intake on bacterial glycolytic activity, end products of fermentation and bacterial steroid transformation were studied in rats associated with a human faecal flora. Rats were fed a human-type diet containing 0, 5 or 10% TOS. Caecal pH decrease correlated with the amount of TOS in the diet. Intake of the TOS diet induced a decrease in blood cholesterol and a strong increase in beta-galactosidase activity in the hindgut. TOS fermentation led to production of hydrogen and short chain fatty acids, whereas ammonia and branched-chain fatty acids were decreased. A diet containing 10% TOS increased caecal lactic acid concentrations and reduced beta-glucuronidase activities and steroid transformation.