Abstract Redox-mediated injury is an important pathway in the destruction of β thalassemic red blood cells (RBC). Because of the autoxidation of the unstable hemoglobin chains and subsequent release of globin free heme and iron, significant amounts of superoxide (O 2 −) and, more importantly, hydrogen peroxide (H 2O 2) are generated intracellularly. Hence, catabolism of H 2O 2 is crucial in preventing cellular injury. Removal of H 2O 2 is mediated via two primary pathways: GSH-dependent glutathione peroxidase or catalase. Importantly, both pathways are ultimately dependent on NADPH. In the absence of any exogenous oxidants, model thalassemic RBC demonstrated significantly decreased GSH levels ( P < 0.001 at 20 h). Perhaps of greater pathophysiologic importance, however, was the finding that the model thalassemic RBC exhibited significantly ( P < 0.001) decreased catalase activity. Following 20 h incubation at 37°C only 61.5 ± 2.9% of the initial catalase activity remained in the α-hemoglobin chain-loaded cells versus 104.6 ± 4.5 and 108.2 ± 3.2% in the control and control-resealed cells, respectively. The mechanism underlying the loss of both catalase activity and GSH appears to be the same in that both catabolic pathways require adequate NADPH levels. As shown in this study, model β thalassemic cells are unable to maintain a normal (∼1.0) NADPH/NADP total ratio and, after 20 h, the model β thalassemic cells have a significantly ( P < 0.001) lower ratio (∼0.5) which is quite similar to a G6PD-deficient RBC. In support of these findings, direct inactivation of catalase gives rise to significantly increased oxidant damage. In contrast, GSH depletion is not closely associated with oxidant sensitivity. Indeed, the consumption of GSH noted in the thalassemic RBC may be via a prooxidant pathway as augmentation of cellular GSH levels actually enhances α-hemoglobin chain-mediated injury.