Apurinic/apyrimidinic (AP) endonuclease (APE; EC 4.2.99.18) plays a central role in repair of DNA damage due to reactive oxygen species (ROS) because its DNA 3′-phosphoesterase activity removes 3′ blocking groups in DNA that are generated by DNA glycosylase/AP-lyases during removal of oxidized bases and by direct ROS reaction with DNA. The major human APE (APE-1) gene is activated selectively by sublethal levels of a variety of ROS and ROS generators, including ionizing radiation, but not by other genotoxicants—e.g., UV light and alkylating agents. Increased expression of APE mRNA and protein was observed both in the HeLa S3 tumor line and in WI 38 primary fibroblasts, and it was accompanied by translocation of the endonuclease to the nucleus. ROS-treated cells showed a significant increase in resistance to the cytotoxicity of such ROS generators as H₂O₂ and bleomycin, but not to UV light. This “adaptive response” appears to result from enhanced repair of cytotoxic DNA lesions due to an increased activity of APE-1, which may be limiting in the base excision repair process for ROS-induced toxic lesions.