The spice principles curcumin (from turmeric) and eugenol (from cloves) are good inhibitors of lipid peroxidation. Lipid peroxidation is known to be initiated by reactive oxygen species. The effect of curcumin and eugenol on the generation of reactive oxygen species in model systems were investigated. Both curcumin and eugenol inhibited superoxide anion generation in xanthine-xanthine oxidase system to an extent of 40% and 50% at concentrations of 75 microM and 250 microM respectively. Curcumin and eugenol also inhibited the generation of hydroxyl radicals (.OH) to an extent of 76% and 70% as measured by deoxyribose degradation. The .OH-radical formation measured by the hydroxylation of salicylate to 2,3-dihydroxy benzoate was inhibited to an extent of 66% and 46%, respectively, by curcumin and eugenol at 50 microM and 250 microM. These spice principles also prevented the oxidation of Fe2+ in Fentons reaction which generates .OH radicals.