Manganese superoxide dismutase (MnSOD) has been previously shown to suppress the malignant phenotype of human melanoma and breast cancer cells. To test the possible role of MnSOD in glioma malignancy, MnSOD was overexpressed in wild type human glioma U118 cells and subcloned U118-9 cells by transfection of human MnSOD cDNA. The MnSOD-transfected cell lines demonstrated expression of exogenous (plasmid) MnSOD mRNA, increase in MnSOD immunoreactive protein, and a three- to eightfold increase in MnSOD enzymatic activity. The MnSOD overexpressing cell lines became less malignant as demonstrated by requiring a higher serum concentration to grow in vitro and much slower tumor growth in nude mice than the parental and neo control cell lines. These findings further support the hypothesis that MnSOD may be a tumor suppressor gene in a wide variety of human tumors.