The uptake of tobacco smoke constituents from gaseous and particulate phases of mainstream smoke (MS), inhaled by smokers, and of environmental tobacco smoke (ETS), breathed in by non-smokers, was investigated in two experimental studies. Tobacco smoke uptake was quantified by measuring carboxyhemoglobin (COHb), nicotine and cotinine in plasma and urine and the data obtained were correlated with urinary excretion of thioethers and of mutagenic activity. An increase in all biochemical parameters was observed in smokers inhaling the complete MS of 24 cigarettes during 8 h, whereas only an increase in COHb and, to a minor degree, in urinary thioethers was found after smoking the gas phase of MS under similar conditions. Exposure of non-smokers to the gaseous phase of ETS or to whole ETS at similar high concentrations for 8 h led to identical increases in COHb, plasma nicotine and cotinine as well as urinary excretion of nicotine and thioethers which were much lower than in smokers. Urinary mutagenicity was not found to be elevated under either ETS exposure condition. As shown by our results, the biomarkers most frequently used for uptake of tobacco smoke (nicotine and cotinine) indicate on the one hand the exposure to particulate phase constituents in smoking but on the other hand the exposure to gaseous phase constituents in passive smoking. Particle exposure during passive smoking seems to be low and a biomarker which indicates ETS particle exposure is as yet not available. These findings emphasize that risk extrapolations from active smoking to passive smoking which are based on cigarette equivalents or the use of one biomarker (e.g. cotinine) might be misleading.