Erythrocyte response to oxidant challenge has been used as an indicator of cell membrane tolerance to oxidant stress. This study describes a simple technique for evaluation of erythrocyte response to hydrogen peroxide oxidant stress using flow cytometry as an assay method. Challenged cells showed detectable increase in fluorescence over the basalevel. This increase in fluorescence was both concentration and time dependent. The test was standadized using 0.1 mM hydrogen peroxide and 2 h incubation. The standardized test showed a higher fluorescence of challenged umbilical cord erythrocytes compared to challenged adult red blood cells. We recommend further use of this test for examining other populations of red blood cells. © 1995 Wiley-Liss, Inc.