Abstract: HMP-3 was isolated from Hypsizigus marmoreus fruit body by boiling water and fractionated by ion -exchange chromatography. HMP-3 was proved to be homogeneous by high performance size exclusion chromatography (HPSEC) . Their monosaccharide levels were significantly different, including the glucose content (80. 18mg/g) and galactose content (10. 82mg/g) . The average molecular weight (Mw) of HMP-3 was (7460±110) ku and its radius of gyration (Rg) was (21. 6±0. 4) nm determined by HPSEC-MALLS-RI system. The backbone of HMP-3 was mainly glucose molecules connected with 1, 4-glycosidic linkages which contained considerable amounts of glucose and/or galactose connected through 1, 6-glycosidic linkages. Little cytotoxicity was apparent in cancer cell lines (AGS, DLD-1, HeLa) from the addition of these polysaccharides;however, they could enhance the proliferation of murine macrophage Raw264. 7 cells and significantly stimulated Raw264. 7 to release nitric oxide (NO) , suggesting their potential immunomodulating activities.