We demonstrate that α-ketoacids reduce and, in some instances, abrogate menadione-induced DNA damage and cytotoxicity in the human breast cancer cell line, MCF7. We confirm that α-ketoacids quench the copious amounts of H2O2 generated by menadione while these α-ketoacids undergo nonenzymatic oxidative decarboxylation; our data thus support enhanced H2O2 production as an important pathway for menadione-induced DNA damage and cytotoxicity. We also demonstrate that α-ketoacids scavenge H2O2 generated by mitochondria and microsomes when these organelles are exposed to menadione; additionally, α-ketoacids protect oxidant-vulnerable enzymes against functional impairment induced by H2O2. Finally, we provide the first in vivo demonstration that acute elevations in concentrations of α-ketoacids in rat tissues and urine scavenge H2O2. We conclude that enhanced H2O2 production is a major pathway for menadione-induced DNA damage and cytotoxicity and that the diverse α-ketoacids present within the cell must he considered, along with glutathione peroxidase and catalase, as part of the intracellular antioxidant defense mechanisms that regulate the ambient levels of H2O2.