In this study, Ecklonia cava was enzymatically hydrolyzed to prepare water-soluble extracts, using five carbohydrases (Viscozyme, Celluclast, AMG, Termamyl, and Ultaraflo) and five proteases (Protamex, Kojizyme, Neutase, Flavourzyme, and Alcalase), and the potential antioxidant activity of each was assessed. The Celluclast and Viscozyme extracts of E. cava evidenced good hydrogen peroxide (H2O2) scavenging activities (73.25% and 72.92%, respectively) as compared to those of other enzymatic extracts. Therefore, the Celluclast enzymatic extract was selected for use in further experiments, and separated into four different molecular weight fractions (< 1, 1-10, 10-30 and > 30 kDa). Among these fractions, the > 30 kDa fraction manifested the most profound H2O2 scavenging activity, with a measured IC50 of 13 mu g/ml. The > 30 kDa fraction also strongly enhanced cell viability against H2O2-induced oxidative damage, and evidenced relatively good lipid peroxidation inhibitory activity in a Chinese hamster lung fibroblast (V79-4) cell line. This fraction also effected a reduction in the proportion of cells undergoing H2O2-induced apoptosis. as was demonstrated by a decreased quantity of sub-G(1) hypodiploid cells and decreased apoptotic body formation on the flow cytometry assay. These results clearly indicate that the > 30 kDa fraction of E. cava possesses good antioxidant activity against H2O2 mediated cell damage in vitro. (c) 2006 Elsevier Ltd. All rights reserved.