Two studies, A and B, were undertaken in order to assess the biological impact of the Environmental Tobacco Smoke (ETS) in nonsmokers. The study A was carried out in a group of 14 nonsmokers prisoners who were each placed, with their agreement, in a cell already occupied by three smokers. A protocol based on a self-administered questionnaire, measurements of respiratory function, tests of urinary mutagenicity and determinations of smoking indicators (carbon monoxide in expired air, nicotine, cotinine and thiocyanates in blood and in urine) was applied to intent to evaluate the possible changes occurred at the end of one month's incarceration. For the chosen parameters no significant difference could be detected at the end of the exposure. The object of the study B was the comparison of biological markers levels in 187 persons working in a Parisian administration who were distributed in three groups according to a self-administered questionnaire: smokers (S), exposed non-smokers (ENS) and nonexposed nonsmokers (NENS). Measured biological markers were carbon monoxide in expired air, carboxyhemoglobin, urinary nicotine and its metabolites, urinary thioethers and mutagens. All the biological indicators differ significantly (p<0.001) in S and NS. Referring to NENS, influence of exposure in ENS is slightly marked for several indicators (COHb, SCN) and more for others (expired CO, nicotine and its metabolites, mutagens and thioethers) even if the differences are not significant. If the three subgroups of ENS are considered the mean values of biological markers decrease regularly in accordance with the time elapsed between the last exposure to ETS and the sampling of physiological fluids. A significant difference (p<0.05) was only found for CO in expired air. This result reveals the importance of collecting the biological samples as soon as possible after the exposure to passive smoke for indicators which have a limited half-life.