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The structures and cytotoxic activities of water-soluble polysaccharides were investigated to search for biologically active polysaccharides from the fruit bodies of quinine conks (Fomitopsis officinalis). The decoctions of this medical fungus are actively used in folk medicine in many countries and traditional Chinese medicine. From the fungal extract we prepared, only branched β-glucan had cytotoxic activity among all the water-soluble polysaccharides. This glucan is characterized by a regular structure. Its backbone is formed by 1,3-linked β-D-Glcp residues, of which every third residue is substituted at O-6 by a single β-D-Glcp residue. It has a triple helix conformation according to the data obtained from a colorimetric assay with Congo red dye and is characterized by a high-weight average molar mass (Mw > 800 kDa). β-Glucan possessed cytotoxic activity against HeLa cells (IC 50  = 318 ± 47 μg/mL) and induced the formation of apoptotic bodies around most cancer cells at a concentration of 200 μg/mL. It should be noted that extraction with boiling water, which is usually used to obtain extracts and decoctions, is unable to isolate active β-glucan. Active β-glucan can be obtained in an individual state by cold alkali extraction after dehydration of the fruit bodies and removal of the components extractable by boiling water., (Copyright © 2020 Elsevier Ltd. All rights reserved.)