Abstract: 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, yielding preparations of plasma membrane and mitochondria. In preparations derived from Bcl-2-expressing cells, both fractions contained Bcl-2 protein. 5-NS and 12-NS signals were significantly decreased following Aβ and H2O2 exposure in control PC12 mitochondrial membranes, and Bcl-2 largely prevented these effects. Plasma membrane preparations containing Bcl-2 were also resistant to radical-induced loss of spin label. Collectively, our data suggest that Bcl-2 is localized to mitochondrial and plasma membranes where it can act locally to suppress oxidative damage induced by Aβ and H2O2, further highlighting the important role of lipid peroxidation in apoptosis.