Plant polyphenols are known to possess antioxidant activities. In recent years, there have been numerous reports confirming the efficacy of these compounds to improve plasma antioxidant capacity in humans. Current methods to evaluate the antioxidant potential of polyphenols are based on in vitro assay procedures (TEAC, ORAC, FRAP, DPPH). However, the antioxidant potential assessed by these methods does not correlate with the biological activity observed in vivo. Eukaryotic cells display a plasma membrane redox system (PMRS) that transfers electrons from intracellular substrates to extracellular electron acceptors. Here, we describe a method to evaluate the antioxidant potential of plant polyphenols based on their ability to enter the erythrocytes and donate electrons to PMRS. We also present results to show the potentiating effect of quercetin, EGCG, EC and catechin on erythrocyte PMRS activity.