Fraction (PMEx-AH-${eta}$) of water-soluble polysaccharide, showing stimulating activity against macrophages, was isolated from mycelia of solid cultured Phellinus linteus by hot-water extraction, ethanol precipitation, and chromatography. Chemical characteristics of PMEx-AH-${eta}$ were as follows: carbohydrate content, 71%; monosaccharide composition, Man:Glu:Gal (9:64:27); molecular weight, $1-7{ imes}10^4$; uronic acid content, 6.8%. Fundamental structure of PMEx-AH-${eta}$ is deduced as ${eta}$-($1{$ ightarrow}6$)-D-glucan with ${eta}$-($1{ ightarow}3$)-D-glucosidic side chains based on methylation analysis.