Measurements of 15 nonsmokers and 3 smokers breathing environmental tobacco smoke (ETS), were conducted to study particle deposition within the human respiratory tract. The subjects inhaled ETS of count median diameter (CMD) of about 0.2 micron and geometric standard deviation (GSD) of 1.7 The particle size distribution in the submicrometer range in the inhaled and exhaled air from the subjects was measured using a scanning mobility particle sizer (SMPS). A deposition of 56.0 +/- 15.9% was observed for nonsmokers while breathing ETS through the nose and 48.7 +/- 11.6% while breathing ETS through the mouth. One individual tested four times gave an average deposition of 57.4 +/- 11.5%, providing an indication of intraindividual variation. Such a variation is expected since the breathing rate was not controlled in order that an indication of the deposition experienced on a day-to-day basis could be obtained. For nonsmokers the deposition while breathing through the mouth was lower than through the nose and the variability within the measurements was also lower for mouth breathing. The latter could be due to the variation in individual size and shape of the nasal passage. Smokers had, on average, a higher rate of deposition but also a higher interindividual variability making it difficult to draw conclusions with respect to the affect of smoking on ETS particle deposition. The average deposition of the three smokers was 65.3 +/- 24.1% for nasal breathing and 66.1 +/- 17.6% for mouth breathing.