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The bcl‐2 protooncogene product possesses antiapoptotic properties in neuronal and nonneuronal cells. Recent data suggest that Bcl‐2's potency as a survival factor hinges on its ability to suppress oxidative stress, but neither the subcellular site(s) nor the mechanism of its action is known. In this report electron paramagnetic resonance (EPR) spectroscopy analyses were used to investigate the local effects of Bcl‐2 on membrane lipid peroxidation. Using hydrogen peroxide (H₂O₂) and amyloid β‐peptide (Aβ) as lipoperoxidation initiators, we determined the loss of EPR‐detectable paramagnetism of nitroxyl stearate (NS) spin labels 5‐NS and 12‐NS. In intact cell preparations and postnuclear membrane fractions, Aβ and H₂O₂ induced significant loss of 5‐NS and 12‐NS signal amplitude in control PC12 cells, but not PC12 cells expressing Bcl‐2. Cells were subjected to differential subcellular fractionation …