Aqueous extracts of cotton dust and bract produced a reproducible, reversible and dose-dependent bronchoconstriction in the airways-perfused isoLated Lung of the guinea pig. On the basis of the effect of various inhibitors of the arachidonic acid cascade, it was concluded that the extracts of both materials activated phospholipase A(2). In the case of cotton dust, this led to the formation of Lipoxygenase products, white for bract, metabolites of the cyclo-oxygenase pathway were formed. Modification of the composition of the perfusion fluid affected the bronchoconstrictor activity of both dust and bract and demonstrated that the effects of these extracts were dependent to some extent on the intra- and extra-cellular concentrations of calcium ions.