The carcinogenic mechanism of smoking is related with the production of electrophilic reactants and their possible covalent binding with DNA. On the other hand, there are detoxifying mechanisms such as glutathione-S-transferase, which results in mercaptopuric derivatives that are excreted in the urine. The integrity of the erythrocyte membrane is maintained by reduced glutathione among other factors. In the present study, the concentrations of erythrocyte reduced glutathione (GSH) and urinary thioethers (UT) were measured in a sample of 81 subjects divided in two groups. Group I: 30 nonsmokers; group II: 51 smokers, subdivided in group IIA (26 individuals smoking 10-20 cigarettes/day) and group IIB (25 individuals smoking more than 20 cigarettes/day). In the present study, the usefulness of GSH and UT as markers of collective internal contamination and of individual risk regarding tobacco exposure were evaluated. A higher concentration of GSH was found in smokers than in nonsmokers (F = 6.84, p less than 0.02). Regarding VT elimination, a significant increase in these parameters was found in association with the grade of smoking (p less than 0.05). They were higher in the subjects from the subgroup IIB than in the subgroup IIA (moderate smokers).