Manganese(II) has been shown to exhibit catalase-like activity under physiological conditions. In the course of studies to test the antioxidant activity of Mn(II) on HeLa cells, it was observed at high concentrations (1-2 mM) that Mn(II) also induced apoptosis, as judged by changes in cell morphology, caspase-3 activation, cleavage of poly(ADP) ribose, and DNA condensation. However, in contrast to established mechanisms, the Mn(II)-induced apoptosis is associated with an increase rather than a decrease in mitochondrial inner-membrane potential, as monitored by the fluorescent probe tetramethylrhodamine ethyl ester. Based on immunochemical analysis, Mn(II)-induced apoptosis does not lead to the release of cytochrome c into the cytosol. These and other measurements show that treatment with Mn(II) leads to enhancement of the mitochondrial "membrane mass," has no effect on mitochondrial volume, and does not affect the permeability transition pore. Together, these results support the view that Mn(II)-induced apoptosis occurs by a heretofore unrecognized mechanism. In addition, it was demonstrated that Mn(II) treatment leads to an increase in the production of reactive oxygen species (peroxides) and to the induction of the manganese superoxide dismutase and catalase activities but has no effect on the Cu,Zn-superoxide dismutase level.