Two water-soluble extracellular polysaccharides, ETW1 and ETW2, were isolated from the marine bacterium Edwardsiella tarda by ion-exchange and size-exclusion chromatography, and their structures were investigated. ETW1 and ETW2 are mannans, with molecular weights of about 29 and 70 kDa, respectively. The main chain of the polysaccharides consists of (1 -> 3)-linked mannose residues, and branched mannose linkages were also detected. The branch points are located at the C-2 and C-6 positions of the (1 -> 3)-linked mannose residues. The side chains are composed of (1 -> 2)-linked mannose residues and (1 ->)-linked mannose residues. Antioxidant properties of the two extracellular polysaccharides were evaluated with hydroxyl and 1,1-dipheny1-2-picrylhydrazyl (DPPH) radicals scavenging activities and lipid peroxidation inhibition in vitro, and results showed that ETW1 and ETW2 had good antioxidant and hydroxyl and DPPH radicals scavenging activities. ETW1 exhibited higher antioxidant activity than ETW2, and could be a potential source of antioxidant and used as possible food supplement or ingredient in the pharmaceutical industry. (C) 2010 Elsevier Ltd. All rights reserved.