The process of aerosol deposition on indoor surfaces has implications for human exposure to particulate contaminants of both indoor and outdoor origin. In the radiological context, current accident models assume a uniform Dose Reduction Factor (DRF) of 0.5 for indoor residence during the outdoor passage of a radioactive aerosol cloud. To examine the general validity of this figure, indoor aerosol deposition have been determined in four houses. Monodisperse aerosol particles, with mass median aerodynamic diameters (MMAD) in the range 0.5-5.5 mu m, were labelled with neutron-activatable tracers and dispersed in unfurnished and furnished rooms; the decay rate of the particles was then inferred from analysis of sequential air samples. Allowing for the differences in furnishing and level of occupancy between the tests, consistent aerosol deposition velocities were determined and, for furnished rooms, the following empirical expression, relating the particles' MMAD (d(p), in mu m) to the indoor deposition velocity (upsilon(d), in 10(-4) m s(-1)) was derived: upsilon(d) = 0.48 + 0.60 d(p) (r = 0.93). Using this formula, particle size-specific DRFs were determined and found to be in good agreement with previous measurements of I/O ratios for fine and coarse particles. It was concluded that, for realistic dose estimates, a radioisotope-specific factor may be merited. (C) 1997 Elsevier Science Ltd.