An extracellular polymer (exo-polymer) with hepatoprotective properties was produced after a 6-day submerged mycelial culture of Ganoderma lucidum WK-003. The glutamic pyruvic transaminase (GPT) activities in the serum of intoxicated Sprague-Dawley rats were decreased from 871 to 263 by the oral administration of the exo-polymer (20 mg/kg/day) for 4 consecutive days. Rhamnose, arabinose, xylose, mannose, galactose, and glucose were found in the exo-polymer along with aspartic acid, glutamic acid, histidine, serine, glycine, arginine, alanine, tryptophan, valine, phenylalanine, isoleucine, and leucine.