It has recently been postulated that dietary fibre acts as a substrate for colonic flora, and that the resultant microbial growth bulks the faeces. Antimicrobial therapy was used in this study to assess the effect of reduction in colonic microbial proliferation on faecal output in human subjects on a constant dietary fibre intake. Six healthy young male subjects were maintained on constant daily diets and metronidazole (1 g/day) and ampicillin (1 g/day) were administered in divided doses for one week after an initial baseline study period of two weeks. After antimicrobial therapy, mean faecal weights rose from 176.0 +/- 27.0 g to 348.1 +/- 37.7 g/day. Faecal solids increased from 32.9 +/- 4.2 g to 46.1 +/- 5.8 g/day. Faecal neutral detergent fibre increased from 1.92 +/- 0.42 g to 15.19 +/- 2.58 g/day. The mean transit times and mean daily faecal nitrogen remained the same, both before and after treatment. Substantial breakdown of dietary fibre occurs in the human colon which may decrease faecal bulk, suggesting that water holding by dietary fibre is probably of greater importance for faecal bulking.