Overexpression of manganese superoxide dismutase (MnSOD) has been postulated as one possible mechanism of protection from oxidative damage and free radicals. Doxorubicin treatment induces oxygen free radicals, leading to cytotoxicity and myelosuppression. The present study was performed to determine whether over-expression of MnSOD may play a role in resistance to doxorubicin. Retroviral constructs having the human MNSOD gene in the sense orientation and the neomycin phosphotransferase gene (NEOR) as a selectable marker were transduced into the human melanoma cell line A375 and the human histiocytic lymphoma cell line U937. Stably transduced A375 and U937 cells were subjected to 10-100 ng/ml doxorubicin for 24 h and compared with doxorubicin-treated A375 and U937 cells transduced with vector only. A colony forming assay was used to determine cell viability in semi-solid medium. Results demonstrated that wild-type A375 and U937 cells display low levels of endogenous MnSOD mRNA and protein, and are sensitive to doxorubicin treatment. In contrast, A375 and U937 cells transduced with the MNSOD gene consistently demonstrate increased colony formation in the presence of increasing concentrations of doxorubicin. MnSOD-transduced A375 and U937 cells also demonstrate increased MnSOD mRNA and protein levels when compared with wild type or those cells transduced with vector only. These results indicate that overexpression of MnSOD can enhance resistance to doxorubicin treatment.