Aim To isolate, purify, and chemically characterize a polysaccharide showing inhibition of the six-?-helix bundle formation of HIV-1 envelope glycoprotein gp41 from cultured broth of a streptomyces sp.strain. Methods Ethanol was used to precipitate polysaccharides and macromolecules from the broth.Proteins in the precipitate were removed by sevage method.Purification was carried out by DEAE-Cellulose and sephadex G-25 column chromatography. The chemical structure of the polysaccharide was determined with the combined application of HPLC,UV,IR and 1H-NMR spectroscopy, and the methods of periodate oxidation and Smith degradation,etc. Activity of anti-HIV-1 ~gp41 six-?-helix bundle formation was assayed withsandwich ELISA method.Results The purified polysaccharide,designated as SMP for Streptomyces polysaccharide,is neutral with a molecular weight of approximately 4855 Daltons. Sugar analysis showed SMP contains glucose and fructose residues in an approximate molar ratio of 22∶1 (10.96 to 0.48). The glycosidic linkages were estimated to be (1→4)-?-D-pyranoside as its main chain, and 1→6 linkage was attached to the main chain. Activity analysis revealed SMP markedly inhibited the six-?-helix bundle formation of HIV-1 glycoprotein gp41 and the IC_~50 was (145.48?7.25) mg?L~-1 .Conclusion Streptomyces polysaccharide SMP showing inhibition of the six-?-helix bundle formation of HIV-1 envelope glycoprotein gp41 was isolated and purified.