Fruits and vegetables provide dietary fibre some of which is partly soluble in the upper gut; in the colon it is highly fermentable. Using alcohol-insoluble residues prepared from a range of fruits and vegetables the effects of fermentation on the changes in composition and binding capacity have been assessed for the hydrophobic mutagen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx). Fermentation was extensive and resulted in destruction of most of the pectic polysaccharides. Of the unfermented vegetable fibre only cabbage had a measurable hydrophobic binding capacity. The binding capacities of unfermented apple, carrot and sugar beet were negligible. After fermentation, binding capacities, (per mg of fermented residue), increased. Although fermented cabbage was found to have the highest capacity of the fruit and vegetable fibres this remained less than the least effective of unfermented wheat bran samples which had a relatively high affinity for MeIQx. Mucin inhibited the binding of MeIQx to wheat bran fibre but apple fibre did not. The results show that the contribution of fruit and vegetable fibre to a hydrophobic binding matrix in the colon is insignificant and the suggested harmful effect of fruit and vegetable fibre, maintaining hydrophobic mutagens in solution, can be prevented by the presence of wheat bran fibre.