A series of experiments and evaluations of fractions isolated from psyllium seed husk (PSH) were used to test the overall hypothesis that a gel-forming component of PSH is not fermented and that it is this component that is responsible for the laxative and cholesterol-lowering properties of PSH. A gel is isolated from human stools collected during a controlled diet study when PSH is consumed but not when the control diet only is consumed. Evaluations of three fractions isolated from PSH suggest that gel-forming fraction B, which is about 55% of PSH, is poorly fermented and is the component that increases stool moisture and faecal bile acid excretion, the latter leading to lower blood cholesterol levels. Fraction C, representing < 15% of PSH, is viscous, but is rapidly fermented. Fraction A is alkali-insoluble material that is not fermented. In concentrations comparable with their presence in PSH, fractions A and C do not alter moisture and bile acid output. The active fraction of PSH is a highly-branched arabinoxylan consisting of a xylose backbone and arabinose- and xylose-containing side chains. In contrast to arabinoxylans in cereal grains that are extensively fermented, PSH possesses a structural feature, as yet unidentified, that hinders its fermentation by typical colonic microflora.