To determine if constituents of cotton plants might play a role in byssinosis by injuring pulmonary epithelium, we added extracts of cotton dust, green bract, and field-dried bract to human A549 and rat type II pneumocytes. Injury was measured as pneumocyte lysis and detachment, and inhibition of protein synthesis. Extracts of cotton dust and field-dried bract produced significant dose- and time-dependent lysis and detachment of both target cells, while green bract extract was less damaging. Extracts treated with polyvinylpolypyrrolidone to remove tannins produced significantly less injury. In contrast, purified 5,7,3',4'-tetrahydroxy-flavan 3,4-diol (THF), a tannin in cotton dust and bract, caused substantial cell damage. Field-dried bract extract and THF also produced dose-dependent inhibition of pneumocyte protein synthesis. Endotoxin levels did not correlate with observed injury. THF added to rat tracheal explants caused epithelial disruption and desquamation, whereas endotoxin did not. Instillation of cotton dust and field-dried bract extract in rat lungs produced disruption of bronchial epithelium and smooth muscle constriction, while polyvinylpolypyrrolidone-treated cotton dust extract produced no injury. These findings suggest that extracts of cotton plants are toxic to alveolar, tracheal, and bronchial epithelium and that THF or other tannins may be the responsible agents.