Mycelia of Antrodia cinnamomea were extracted with chloroform and hot water. A neutral polysaccharide named ACN2a separated from the water extract was purified using 10% CCl₃COOH, and repeated column chromatography on HW-65 and DE-52 cellulose. Its structure was determined by chemical and spectroscopic analyses. ACN2a was composed of Gal, Glc, Fuc, Man and GalN (in the ratio 1 : 0.24 : 0.07 : 0.026 : faint), in which an α-D-(1→6)-Gal linkage accounted for 73% of all linkages. The ratio of branch points was about 16% of the total residual numbers, and branches were attached to C-2 of galactosyl residues of the main chain. ACN2a had an average molecular weight of 12.9×10⁵ Daltons, [α]_D²⁵=+115° (c=0.44, H₂O); [η]=0.0417dl·g⁻¹, Cp=0.2663 cal/(g·°C). The hepatoprotective effect of ACN2a was evaluated using a mouse model of hepatic injury that was induced by Propionibacterium acnes (P. acnes) and lipopolysaccharide (LPS). The administration of ACN2a (0.4, 0.8 g/kg/d, p.o.), significantly prevented increases in serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) enzyme activities in mice treated with P. acnes–LPS, indicating hepatoprotective activity in vivo.