The in vitro antioxidant activities of the following six sulfated polysaccharides were investigated: iota, kappa and lambda carrageenans, which are widely used in the food industry, fucoidan (homofucan) from the edible seaweed Fucus vesiculosus and fucans (heterofucans) F0.5 and F1.1 from the seaweed Padina gymnospora. With respect to the inhibition of superoxide radical formation, fucoidan had an IC(50) (the half maximal inhibitory concentration) of 0.058 mg.mL(-1), while the IC(50) for the kappa, iota and lambda carrageenans were 0.112, 0.332 and 0.046 mg.mL(-1), respectively. All of the samples had an inhibitory effect on the formation of hydroxyl radicals. The results of peroxidation tests showed that fucoidan had an IC(50) of 1.250 mg.mL(-1) and that the kappa, iota and lambda carrageenans had an IC(50) of 2.753 and 2.338 and 0.323 mg.mL(-1), respectively. Fucan fractions showed low antioxidant activity relative to fucoidan. These results clearly indicate the beneficial effect of algal polysaccharides as antioxidants.