The thiol N-acetylcysteine (NAC), a precursor of intracellular glutathione (GSH), efficiently prevented the induction of lung tumors in Swiss albino mice, when supplemented to the diet (0.2%) both before and after an i.p. injection of the carcinogen urethan (ethyl carbamate). Irrespective of urethan administration, NAC also significantly enhanced GSH S-transferase activity in liver preparations of the same animals. These data show that, under certain conditions, it is possible to prevent chemically induced cancer by increasing the levels of physiological trapping agents.