Typical air movements in many indoor living and working places are as low as, or less than, 0.1 m/s, and so conditions might best be described in terms of calm, as opposed to moving air-or perhaps some combination of the two. However, the history of aerosol sampling science has been dominated by studies of the aspiration efficiencies of sampling devices in moving air, and there have been very few definitive studies of calm air sampling, even for very simple sampling systems. With this in mind, this paper sets out the basis for the development of a larger body of experimental work, and describes an experimental method by which unambiguous measurements may be made for a range of simple aerosol sampling systems (i.e., shape and orientation). The method, referred to as the "direct method"', owes much to the 'thread-of-droplets' approach proposed by Lipatov ct al. tJ. Aerosol Sci. 17 (1986) 763) for studies of aspiration efficiency in moving air. In the new method, monodisperse large droplets of olive oil with aerodynamic diameter in the range from 40 to 70 mum, were, generated using a vibrating orifice aerosol generator and the experimental conditions were controlled very carefully so that a well-defined, vertically-falling droplet stream was obtained. By the application of appropriate tracking apparatus, visual observation of this droplet stream was used to map the pattern of particle trajectories near a given sampling inlet and to identify limiting particle trajectories in order to calculate aspiration efficiency. In an initial application of the new method, measurements of aspiration efficiency were carried out for upwards-facing thin-walled sampling tubes, and the results were found to be in satisfying agreement with the numerical results of Agarwal and Liu (1980, in the Am. Ind. Hyg. Assoc. J. 41 (1980) 191) and the numerical and experimental results of Yoshida et al. (Kagaku Koqaku Ronbunshu 4 (1978) 123). The results of this initial study provide confidence that the method described may now be applied to a wider range of sampling systems.