HMP-1 was isolated from Hypsizigus marmoreus fruit body and fractionated by ion-exchange chromatography.The carbohydrate content of HMP-1 was 99%.HMP-1 was proved to be homogeneous by high performance size exclusion chromatography(HPSEC).Their monosaccharide levels were significantly different,including the glucose content(67.36mg/g) and galactose content(33.36mg/g).The average molecular weight(Mw) of HMP-1 was(4 781±0.6)kDa and its radius of gyration(Rg) was(17.1±1.6)nm determined by HPSEC-MALLS-RI system.The backbone of HMP-1 was mainly glucose molecules connected with 1,4-glycosidic linkages which contained considerable amounts of 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.