The effects of aqueous extracts of cotton, flax and hemp dusts on normal human respiratory epithelium over 4 hours were examined by measuring ciliary beat frequency and by electron microscopy. All three extracts slowed ciliary beat frequency and caused epithelial disruption in a dose-dependent manner. At a concentration of 5mg/mi., cotton dust caused small changes in beat frequency (13 8% ciliary slowing), while flax and hemp (53.9 and 89.5% respectively) 'were more potent. These actions were not related to the endotoxin content of the extracts. Progressive ciliary slowing occurred before epithelial disruption but dyskinesia was not seen. The activity of hemp and flax was heat stable and was mostly removed by dialysis. Although gel filtration separated a high molecular weight fraction with some activity, the majority of the ciliary inhibitory activity was present in the fraction below 1.3kDa.