Fibre, more than any other dietary component, affects human large bowel function, causing an increase in stool output, dilution of colonic contents, a faster rate of passage through the gut and changes in the colonic metabolism of minerals, nitrogen and bile acids. (Fibre here refers to 'dietary fibre', which comprises plant cell wall polysaccharides and lignin, and not to 'crude fibre'.) It is thought that these changes are brought about by fibre passing through the gut undigested and holding water within its cellular structure. Although the amount of water taken up in vitro varies for different types of fibre, this does not correlate in the expected way with the effects these materials have on colonic function. This is because fibre is extensively degraded in the gut, probably by the colonic microflora. Using a newly developed method modified from ruminant nutrition for isolating bacteria, we have shown that the main component of human faeces is bacteria. We have shown that the main component of human faeces is bacteria. We show here that the way in which two contrasting types of dietary fibre act in the colon depends on the extent to which they are digested. Cabbage fibre, which is extensively broken down, provides a readily usable substrate for the stimulation of microbial growth, whereas wheat fibre remains largely undigested and retains water in the gut lumen.