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A gray and low viscosity extracellular polysaccharide (EPS) composed of N-acetylglucosamine, xylose, and mannose was isolated from culture medium of Bacillus sp. strain LBP32 by ethanol precipitation followed by dialysis and freeze-drying. The crude biopolymer showed an apparent molecular weight (M(w)) of similar to 9.62 x 10(4). Chemical and spectroscopic studies revealed that the bacterial biopolymer was composed of a beta-1,4-linked backbone carrying a low content of beta-1,3-linked backbone. In addition, the EPS demonstrated a high antioxidant activity in a concentration dependent manner. The 50% inhibition concentration (IC(50)) for quenching hydroxyl radical (center dot OH) and superoxide radical (center dot O(2)(-)) were 0.042 and 0.165 mg/mL, respectively. Furthermore, the EPS demonstrated a strong protective effect against lipid peroxidation and radiation such as UV radiation and ion beam irradiation. These results indicate that the protective effects of the EPS were most likely due to its free radical scavenging ability.