A method has been developed, based on techniques used for isolating bacteria from the rumen, that enables human faeces to be fractionated into three major components. The method requires repeated, vigorous agitation of a suspension of faecal solids with detergent, the use of a stomacher, and high-speed centrifugation. By this means the faecal microflora are separated from faecal dietary-fibre residues. These two components, with water-soluble material in the stool, comprise 98.3 +/- 0.9% of faecal solids. The purity of the microbial fraction was demonstrated by Gram and plant stains and by scanning electron microscopy. Microscopic counts of the bacteria in each fraction of the stool showed that the microbial fraction contained 95% of the total bacteria. Chemical analysis of the component sugars indicated 6-7% possible contamination by non-bacterial polysaccharides. The bacterial pellet was 6% nitrogen, and accounted for 60% of the total faecal nitrogen. When faeces from nine healthy subjects on a metabolically controlled British-type diet were studied, bacteria comprised 54.7 +/- 1.7% of the total solids, fibre 16.7 +/- 0.8% and soluble material 24.0 +/- 1.3%. Bacteria therefore represent a much larger proportion of the faecal mass than was previously thought.