A Total Oligomers Flavonoids (TOFs) and ethyl acetate extracts of Cyperus rotundus were analyzed, in vitro, for their antioxidant activity using several biochemical assays: the xanthine (X)/xanthine oxidase (XO), the lipid peroxidation induced by H(2)O(2) in K562 human chronic myelogenous leukemia cells and the DNA damage in pKS plasmid DNA assay induced by H(2)O(2)/UV-photolysis and for their apoptotic effect. TOF and ethyl acetate extracts were found to be efficient in inhibiting xanthine oxidase with IC(50) values of 240 and 185 microg/ml and superoxide anion with IC(50) values of 150 and 215 microg/ml, respectively. Also, all the extracts tested were effective in reducing the production of thiobarbituric acid reactive substances (TBARS) and were able to protect against H(2)O(2)/UV-photolysis induced DNA damage. The highest activity, measured as equivalents of MDA concentration, was observed in the ethyl acetate extract (MDA=2.04 nM). In addition, the data suggest that only TOF enriched extract exerts growth inhibition on K562 cells through apoptosis induction. Therefore, these extracts were subjected to further separation by chromatographic methods. Thus, three major compounds (catechin, afzelechin and galloyl quinic acid) were isolated from the TOF enriched extract and five major compounds (luteolin, ferulic acid, quercetin, 3-hydroxy, 4-methoxy-benzoic acid and 6,7-dimethoxycoumarin) from ethyl acetate extract. Their structures were determined by spectroscopic data analysis and comparison with the literature. In addition, we evaluate the biological activities of the catechin, ferulic acid and luteolin. This investigation has revealed that the luteolin was the most active in reducing the production of TBARS (MDA=1.5 nM), inhibiting significantly the proliferation of K562 cells (IC(50)=25 microg/ml) and protecting against H(2)O(2)/UV-photolysis induced DNA damage. In conclusion, the study reveals that the ability of C. rotundus to inhibit the enzyme xanthine oxidase (XO), the lipid peroxidation and to exert apoptotic effect, may explain possible mechanisms by which C. rotundus exhibits its health benefits.