Treatment of the radiation-sensitive CHO mutant, xrs-5, with the demethylating agent 5-azacytidine results in the complete conversion to wild-type levels of X-ray resistance in 50% of the colonies examined (10/20). In addition to being sensitive to X rays, xrs-5 is also sensitive to the killing effects of the crosslinking agent cisplatin. The 5-azacytidine-treated xrs-5 cells which exhibit wild-type survival levels following exposure to X rays failed to demonstrate conversion to wild-type levels of resistance to cisplatin. These results support the hypothesis that increases in gene expression can alter the radioresistance of xrs-5 cells without influencing the cells' survival after exposure to cisplatin.