In order to reduce the haemolytic susceptibility of glucose-6-phosphate dehydrogenase (G-6-PD) deficient erythrocytes, Ginkgo biloba extract (EGb 761) and vitamin E (vit E) were used as antioxidant agents and their effects compared. The erythrocyte suspensions from control and G-6-PD deficient patients were subjected to hydrogen peroxide (H2O2) incubation for 1 h. The produced thiobarbituric acid reactive substance (TBARS) levels measured as nmol/g Hb were compared with those of the erythrocytes administered 250 microg/mL EGb 761 or vit E previously or concomitantly with H2O2. Preincubation with EGb 761 reduced the TBARS levels from 317.14 +/- 25.27 to 160.09 +/- 21.97 nmol/g Hb in controls and from 348.24 +/- 7.79 to 205.60 +/- 14.22 nmol/g Hb in deficient erythrocytes. Concomitant application of EGb 761 with H2O2 resulted in similar but less reduction. The antioxidative effects of vitamin E were comparable to those of EGb 761. Contrary to the results obtained from oxidant conditions, the antioxidant characteristics of EGb 761 and vitamin E were not observed when they were applied directly to the erythrocytes without oxidative stress. The findings demonstrate that irrespective of administration time, EGb 761 significantly reduced TBARS levels in the erythrocytes of control and G-6-PD deficient patients subjected to oxidative stress.