The effect of daily inhalation of aqueous extracts of cotton dusts were examined in the respiratory ciliated epithelium in guinea pig by electron microscopy. Following five days exposure, there was a significant loss of cilia, cellular disruption, detachment from basal membrane and swollen mitochondria compared to control. Extrusion of cellular components were also observed. This indicates that exposure of the guinea pig to cotton dust extract causes marked cellular damage to the airway epithelium. This toxic reaction may have an important role in the development of byssinosis.