Guar gum from four industrial sources was investigated. The viscosity of two preparations of hydrated guar gum in the form of powdered flour and one granulate flour was measured at 22 degrees and 32 degrees C and pH 1.0 and pH 4.0. Viscosity measurements on wax-coated guar granules proved impossible but visual assessment indicated an extremely low viscosity in all conditions. These findings were compared with the ability of the equivalent of 5 g guar gum of the various preparations to modify the absorption of a 50 g liquid glucose load. The mean post-prandial blood glucose curve was not significantly different from the control situation after the incorporation of each preparation. Despite the granulate flour attaining a considerably lower viscosity than the powdered flour they were equally effective in significantly reducing the mean post-prandial insulin curve (area under the curve (0-180 min) reduced by 46 and 50% respectively). The wax-coated granules which achieved minimal viscosity caused significantly less reduction of post-prandial insulin levels (area under the curve reduced by 37%). The viscosity of guar gum upon hydration is of importance in assessing the efficacy of a preparation in clinical use.