2,6-di(2,3-epoxypropoxy)xanthone (EPX), a newly synthesized xanthone derivative, is a potent antitumor agent, which is more cytotoxic than the antitumor drug mytomycin C. EPX also demonstrated stronger growth inhibition to T24 (bladder carcinoma with Ha-ras gene mutation) and 212 cells (a NIH/3T3 derivative, transformed by Ha-ras oncogene) than to PLC/PRF/S (hepatoma with normal Ha-ras gene) and NIH/3T3 cells. The preferential repression of EPX on the cell proliferation of 212 and T24 cells was further demonstrated by decreasing Ha-ras oncogene expression levels while EPX dosage increased. The drug concentrations for 50% inhibition (IC50) of cell growth, DNA synthesis Ha-ras oncogene expression and colony formation of T24 and 212 cells are in the same range and lower than the values for RNA and protein synthesis. Moreover, EPX irreversibly reversed 212 cell morphology from a transformed phenotype to a normal one. These data indicate that EPX probably suppresses tumor cell proliferation by inhibiting DNA synthesis and reverses the transformed properties by suppressing Ha-ras gene expression. The mechanisms of biochemical action and cytotoxicity of EPX remain to be determined. However, our data suggest that the EPX-mediated inhibition of cell proliferative capacity of 212 and T24 cells was preceded by a selective down-regulation of Ha-ras oncogene RNA levels. EPX may have the potential to be used broadly against diverse tumors or specifically against Ha-ras oncogene initiated malignancy.