AbstractLentinan (β‐(1→3)‐D‐glucan) was found to be successfully fractionated by the mixture of dimethyl sulfoxide (DMSO) and lithium chloride (LiCl) as a solvent and acetone as a precipitant. Light scattering and viscosity measurements were made on solutions of fractionated samples in pure DMSO and 0.2M LiCl/DMSO in the range of the molecular weight Mw from 21.7 × 104 to 84.7 × 104. The values of Mw in both solvents were almost the same, but the remarkable difference between the values of intrinsic viscosity [η] demonstrated that the LiCl/DMSO solvent greatly enhances the stiffness of the lentinan backbone. The observed intrinsic viscosity [η] was analyzed by the Yoshizaki‐Nitta‐Yamakawa theory of a worm‐like chain, and the persistence length q and molecular weight per unit contour length ML were determined roughly as 6.0 nm and 890 g nm−1 in 0.2M LiCl/DMSO, and 5.1 nm and 890 g nm−1 in pure DMSO, respectively. This slightly larger persistent length in 0.2M LiCl/DMSO also confirmed the higher stiffness of lentinan enhanced by the LiCl/DMSO solvent. The enhancement of the chain stiffness was ascribed to the electrostatic repulsion because of the hydrogen bonding of the hydroxyl protons of lentinan with the chloride ion, which is in turn associated with the Li+(DMSO)n macrocation complex. © 2012 Wiley Periodicals, Inc. Biopolymers 97: 840–845, 2012.