Inhalation of aqueous extracts of cotton bracts by healthy human volunteers induces reversible lung function changes and symptoms similar to those experienced by cotton textile workers with byssinosis. This airway constrictor response, maximal after 90–120 min, occurred in 95 out of 105 volunteers. This explains why byssinosis is highly prevalent under adverse conditions in cotton textile mills. The airway constrictor activity is retained in purified extracts of the cotton bracts which contain a small molecular (< 1,000 daltons), watersoluble, heat-stable compound.