Measurements of resuspension of 1, 10 and 20 mu m particles from grass swards exposed to three wind speeds within a wind tunnel are reported. Particles used were monodisperse porous silica of about unit density, and were labelled with a stable tracer to ensure that deposited and resuspended amounts could be established by neutron activation analysis using the Imperial College Reactor. Grass swards were allowed to grow to two heights of 10 and 25 cm. Resuspension rates (Lambda) which may be derived in terms of the fraction of the initial deposit that has occurred by a particular time were determined for both sward heights. In another experiment the contaminated grass swards were exposed to a mean free stream wind velocity of 3 m s(-1). In some experiments this wind speed was periodically enhanced by sudden gusts of wind generated artificially by opening and closing a pneumatically operated plate fitted over the tunnel exhaust system. The results of these experiments have indicated that surface and environmental conditions can play a significant role in determining particle resuspension from natural surfaces.