Mangiferin, a C-glucosylxanthone (1,3,6,7-tetrahydroxyxanthone-C2-β-D-glucoside) purified from plant sources was shown to have in vivo growth-inhibitory activity against ascitic fibrosarcoma in Swiss mice. Following in vivo or in vitro treatment, it also enhanced tumor cell cytotoxicity of the splenic cells and peritoneal macrophages of normal and tumor-bearing mice. In vitro treatment of the splenic cells of tumor-bearing mice with mangiferin resulted in augmented killing of tumor cells, both resistant and sensitive to natural killer cells. Mangiferin was also found to antagonize in vitro the cytopathic effect of HIV. The drug appears to act as a potent biological response modifier with antitumor and antiviral effect.