Abstract We have studied erythrocyte Ca 2+-ATPase as a model target for elucidating effects of activated oxygen on the erythrocyte membrane. Either intracellular or extracellular generation of activated oxygen causes parallel decrements in Ca 2+-ATPase activity and cytoplasmic GSH, oxidation of membrane protein thiols, and lipid peroxidation. Subsequent incubation with either dithiothreitol or glucose allows only partial recovery of Ca 2+-ATPase, indicating both reversible and irreversible components which are modeled herein using diamide and t-butyl hydroperoxide. The reversible component reflects thiol oxidation, and its recovery depends upon GSH restoration. The irreversible component is largely due to lipid peroxidation, which appears to act through mechanisms involving neither malondialdehyde nor secondary thiol oxidation. However, some portion of the irreversible component could also reflect oxidation of thiols which are inaccessible for reduction by GSH, since we demonstrate existence of different classes of thiols relevant to Ca 2+-ATPase activity. Activated oxygen has an exaggerated effect on Ca 2+-ATPase of GSH-depleted cells. Sickle erythrocytes treated with dithiothreitol show a heterogeneous response of Ca 2+-ATPase activity. These findings are potentially relevant to oxidant-induced hemolysis. They also may be pertinent to oxidative alteration of functional or structural membrane components in general, since many components share with Ca 2+-ATPase both free thiols and close proximity to unsaturated lipid.