Background : Anticancer drugs act by increasing intracellular hydrogen peroxide levels. Mangafodipir, a superoxide dismutase (SOD) mimic with catalase and glutathione reductase activities, protects normal cells from apoptosis induced by H ₂ O ₂ . We investigated its and other oxidative stress modulators' effects on anticancer drug activity in vitro and in vivo. Methods : Cell lysis and intracellular reactive oxygen species levels were assessed in vitro in human leukocytes from healthy subjects and in murine CT26 colon cancer cells. Cells were exposed to the chemotherapeutic agents paclitaxel, oxaliplatin, or 5-fluorouracil, either in the presence or absence of mangafodipir and other oxidative stress modulators. Cell viability was evaluated by the methylthiazoletetrazolium assay. The effects of mangafodipir and other oxidative stress modulators on peripheral blood counts and on tumor growth were studied in BALB/c mice that were implanted with CT26 tumors and treated with 20 mg/kg paclitaxel. Survival of BALB/c mice infected with Staphylococcus aureus was also examined by treatment group. Statistical tests were two-sided. Results : In vitro lysis of leukocytes exposed to paclitaxel, oxaliplatin, or 5-fluorouracil in combination with mangafodipir was decreased by 46% (95% confidence interval [CI] = 44% to 48%), 30.5% (95% CI = 29% to 32%), and 15% (95% CI = 10% to 20%), compared with lysis of cells treated with anticancer agent alone. Mangafodipir also statistically significantly enhanced in vitro anticancer drug cytotoxicity toward CT26 cancer cells. In vivo, mangafodipir protected mice against paclitaxel-induced leukopenia. Moreover, the survival rate of mice infected with S. aureus and treated with paclitaxel was higher when mangafodipir was also administered (survival: 3 of 17 versus 14 of 17, P <.001). In addition, mangafodipir amplified the inhibitory effect of paclitaxel on CT26 tumor growth in mice. Conclusions : Mangafodipir decreased hematotoxicity and enhanced cytotoxicity of anticancer agents.