Abstract:A new water-soluble hetero-polysaccharide, APSID3, was obtained from a hot-water extract of the roots of Astragalus membranaceus (Fisch.) Bunge by DEAE-Sepharose Fast Flow and Sephacryl S-300 chromatography. The molecular weight of APSID3 was estimated to be 5.79 × 105 Da. Based on a sugar composition analysis, methylation analysis, partial hydrolysis and 13C nuclear magnetic resonance experimentation, it was concluded that the minimal repeat unit of APSID3 was composed of one terminal arabinose, one 1,5-linked arabinose, one 1,3-linked rhamnose, one 1,3,4-linked rhamnose, five 1,4-linked methyl galacturonates and six 1,4-linked methyl glucuronates.(Author for correspondence. Tel: +86 (0)21 5132 2507; E-mail: zhibihu@hotmail.com)