A novel polysaccharide (GBP50S2) with antioxidant activity was isolated from Ginkgo biloba. The structure of GBP50S2 was elucidated on the basis of physico-chemical and instrumental analyses, and its average molecular weight (Mw=2.30x10(5)) was determined by gel permeation chromatography. The backbone of GBP50S2 was composed of (1-->4)-linked alpha-d-mannopyranosyl residues which branched at O-3. The three branches consisted of beta-l-rhamnopyranosyl residues, (1-->4)-linked alpha-d-galactopyranosyl terminated with beta-l-rhamnopyranosyl residues, and (1-->3,4)-linked alpha-d-mannopyranosyl terminated with beta-l-rhamnopyranosyl residues, respectively. In the in vitro antioxidant assay, GBP50S2 was found to possess DPPH radical-scavenging activity and hydroxyl radical-scavenging activity with an IC(50) value of 0.412 mg/mL and 0.482 mg/mL, respectively.