An aqueous extract was prepared from a sample of airborne dust collected in a Yugoslavian flax mill. Inhalation of the nebulised extract by a panel of four volunteers caused a decrease in specific airway conductance. This effect had a similar time-course to that induced by inhalation of a similarly prepared cotton dust extract. The flax extract was more active than the cotton dust extract on the basis of both whole dust and quantity of aqueous soluble fraction. There were different endotoxin concentrations in doses of the cotton and flax dust extracts that produced a similar magnitude of airway constriction. The broncho constrictor effects of the flax dust extract were antagonised by the calcium channel blocker, verapamil, but unaffected by the cyclooxygenase inhibitor, indomethacin, and the potent selective H(1) receptor antagonists, terfenadine and loratadine. It is suggested that the bronchoconstrictor action of flax dust extract may be mediated by a mechanism similar to that proposed for the effect of cotton dust extract on the airways.