Postprandila glycaemia and rise in serum insulin after carbohydrate-containing meals were reduced by the addition of guar flour or pectin, or both. After a liquid test meal (four subjects) the 30-min blood glucose was reduced from 6.33 +/- 0.19 mmol/litre (114 +/- mg/dl), mean +/- SEM, in the control subjects of 4.77 +/- 0.17 mmol/litre (86 +/- 3 mg/dl) by addition of guar gum (P less than 0.05). The mean insulin level was also significantly lower at 15 min. A breakfast test meal (bread, butter, marmalade, and tea) resulted in a mean 15-min blood glucose of 6.18 +/- 0.21 mmol/litre (111 +/- 4 mg/dl) in eight subjects; 10 g of pectin added to the marmalade reduced this level to 5.64 +/- 0.17 mmol/litre (102 +/- 3 mg/dl) (P less than 0.01). The insulin levels were significantly lower at 15, 30, and 45 min. A similar meal in which guar was added to the bread and pectin to the marmalade resulted in significant reductions of blood glucose at 15 min (P less than 0.002) and 30 min (P less than 0.01). The insulin values were also significantly lower throughout the first 90 min of the test. This action of unavailable carbohydrate may prove useful in the dietary control of diabetes.