The pulmonary clearance in guinea-pigs of an inhaled aerosol of tin-colloid radio-labelled with (99m-)Technetium was monitored over a six-hour period by gamma-scintigraphy. The removal of particles impermeable to the airways epithelium was thus considered a reflection of mucociliary clearance mechanisms. The ability of the lung to clear such particles was examined following inhalation of cotton dust extract over a five-day treatment period. The results hypothesize the involvement of mucociliary clearance as a possible factor in the aetiology of Byssinosis.