Exo-polysaccharide obtained from the submerged cultivation of Ganoderma lucidum mycelium was fractionated. The structural analysis of the acidic exo-polysaccharide fraction (BWS-DA-GI), showing high antitumor activity, was carried out and compared to the mycelial acidic fraction (MWS-DA-GI). The major sugar constituents of the fraction of BWS-DA-GI were glucose, galactose and mannose in the molar ratio of 2.5 : 2.1 : 2.5. The minor components in this fraction were xylose and fucose. While the major sugar constituents of the mycelial acidic fraction of MWS-DA-GI were galactose, fucose, mannose and glucose. The trace components in this fraction was xylose. From the results of periodate oxidation, Smith degradation, affinity chromatography and methylation analysis, the chemical structures of the two fractions, BWS-DA-GI and MWS-DA-GI were both determined as ${eta}-1,3$ glucans. It was also estimated that BWS-DA-GI had a $1{ ightarrow}6$ glucosidic linkage and MWS DA-GI had $1{ ightarrow}4$ and $1{ ightarrow}6$ glucosidic linkages. The molecular weights of these fractions, MWS-DA-GI and MWSDA-GI were estimated as $1.2{ imes}10^6;and;1.0{ imes}10^6$ dalton, respectively.