To investigate hypoglycemic substances from pumpkin, protein-bound polysaccharide was isolated by activity-guided isolation from water soluble substances of the fruits. The protein-bound polysaccharide from pumpkin fruits (PBPP) was identified to consist mainly of polysaccharide (approximately 41.21%) and protein (approximately 10.13%) by anthrone test and Lowry-Folin test. Different doses of PBPP were evaluated for hypoglycemic activity and the effect on serum insulin levels in alloxan diabetic rats. The results indicated that PBPP can obviously increase the levels of serum insulin, reduce the blood glucose levels and improve tolerance of glucose. The hypoglycemic effect of big dose PBPP group (1000 mg/kg body weight) excelled that of small dose PBPP group (500 mg/kg body weight) and antidiabetic agent group. The results suggest that the hypoglycemic effect of PBPP depends on the dose and PBPP possesses the possibility of being developed from a new antidiabetic agent.