The effect of soy polysaccharide on postprandial plasma glucose levels was examined in a crossover experiment involving seven obese noninsulin dependent maturity-onset diabetic patients fed a standard meal without or with 10 g of this fiber source. Postprandial concentrations of plasma insulin, glucagon, pancreatic polypeptide, and somatostatin were measured to explore the mechanism of action. The effect on plasma triglyceride levels was also studied. Supplementation of soy polysaccharide significantly enhanced return of serum glucose levels towards fasting level during the latter half of the meal test. Addition of soy polysaccharide had no effect on plasma insulin levels but appeared (p greater than 0.05) to lessen postprandial increases in glucagon and pancreatic polypeptide levels while it raised somatostatin levels. Soy polysaccharide significantly reduced the rise of postprandial plasma triglyceride levels. The changes in plasma glucagon, pancreatic polypeptide, and somatostatin levels may have been instrumental in the observed postprandial glucose and triglyceride effects.