The study was aimed at evaluating the antioxidant activity of aqueous extract of C islandica. The antioxidant activity, reducing power, superoxide anion radical scavenging and free radical scavenging activities were studied. The antioxidant activity increased with the increasing amount of extracts (from 50 to 500 mug) added to linoleic acid emulsion. About 50, 100, 250, and 500 mug of aqueous extract of C islandica showed higher antioxidant activity than 500 jig of a-tocopherol. The samples showed 96, 99, 100, and 100% inhibition on peroxidation of linoleic acid, respectively. On the other hand, the 500 mug of alpha-tocopherol showed 77% inhibition on peroxidation on linoleic acid emulsion. Like antioxidant activity, the reducing power, superoxide anion radical scavenging and free radical scavenging activities of C. islandica depends on concentration and increasing with increased amount of sample, The results obtained in the present study indicate that C. islandica. is a potential source of natural antioxidant. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.