The ability of three related naturally occurring flavonols in inhibiting Hb oxidation and lipid peroxidation of human erythrocyte membranes was evaluated. The flavonols tested exhibited the following order of potency to inhibit tent-butyl hydroperoxide-induced Hb oxidation: quercetin > rutin > morin. The Hb oxidation was estimated by the extent of metHb and hemichrome formation induced by tertbutyl hydroperoxide. Quercetin or rutin (0.5 mM) increased oxyHb levels about 33% and 10%, respectively. Morin (0.5 mM) was pro-oxidant, decreasing the oxyHb about 7%. Despite the pro-oxidant action on Hb oxidation, morin offered greater protection against lipid peroxidation than rutin, preventing the formation of TBA reactive substances by 33.1%, at 0.2 mM. However, quercetin (0.2 mM) provided approximately 50% protection against TBARS formation. The results show that the flavonoids tested are protective for Hb oxidation and lipid peroxidation on erythrocyte membrane subjected to oxidative stress. Quercetin in particular, may be useful in diminishing oxidative damage to red blood cells.272141152