Using an age-fractionated RBC model, we investigated the in vitro sensitivity of artemether in β-thalassaemic RBC infected with the K1 and FC27 strains of Plasmodium falciparum and, to study the role of oxidant stress in modulating the sensitivity pattern, pro-oxidant (riboflavin) and antioxidant (vitamin E) agents were added to cultures in the presence of artemether. With the FC27 strain, the artemether IC50 doses in thalassaemic samples (whole blood and fractions) were significantly higher compared to equivalent normal RBC samples (P<0·05). However, with the K1 strain, such a significant difference was not demonstrable. The addition of vitamin E reduced the antimalarial effect of artemether in both the FC27 and K1 strains (P<0·0001). In contrast, the addition of riboflavin resulted in a significant increase in antimalarial activity (P<0·0001). This effect of the drug combinations was not influenced by the red cell type (P<0·0001) and there was no interaction between red cell type and drug type (P<0·0001). These findings show that reduced sensitivity to artemether occurs in whole blood and age-fractionated β-thalassaemic trait RBC. It appears that the RBC redox status does not influence the sensitivity to artemether.