The generation of reactive oxygen species has been suggested to occur at increased rates during apoptosis, but the validity and significance of this remain contentious. In several key studies levels of reactive oxygen species have been monitored using the intracellular probe dichlorofluorescin (DCFH2), which undergoes oxidation to the fluorescent dichlorofluorescein (DCF). We report here that cytochrome c, which is released from mitochondria during cell death, is a potent catalyst of DCF formation. In a model system using xanthine oxidase to generate superoxide radicals, the rate of DCF formation was insensitive to changes in the rate of superoxide production over a 17-fold range, but extremely sensitive to nanomolar concentrations of cytochrome c. Thus we conclude that the DCF fluorescence observed in dying cells is a reflection of increased cytosolic cytochrome c. Moreover, we suggest that the suppression of DCF formation by the anti-apoptotic oncoprotein Bcl-2, which has been suggested to have antioxidant properties, can be explained on the basis of its prevention of mitochondrial cytochrome c release,