Benzoic acid (BA) and citric acid (CA) are food additives commonly used in many food products. Food additives play an important role in food supply but they can cause various harmful effects. The in vitro adverse effects of BA and CA and the protective effect of quercetin on human erythrocytes were investigated by measuring malondialdehyde (MDA) levels and superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione-S-transferase (GST) activities. Erythrocytes were incubated with BA and CA, at three doses of 50, 100 and 200 mu g/ml, and quercetin, at a concentration of 10 mu M. After BA and CA application, a dose-dependent increase in MDA level and decreases in SOD, CAT, GST and GPx activities were found in erythrocytes. Among the two food additives, BA exerted a more harmful influence on human erythrocytes than CA. The protective effects of quercetin against oxidative stress induction in the human erythrocytes by CA and BA, were found when these two food additives were applied at each of three doses of 50, 100 and 200 mu g/ml. However, complete protection of quercetin against CA toxicity was only observed when this agent was applied at a lower dose of 50 mu g/ml. Quercetin did not completely protect erythrocytes even at the lowest concentration of BA.