Aqueous extracts of flax and cotton dusts were injected into the perfusion fluid of the isolated perfused lung of the guinea pig. Both flax and cotton dust extracts caused a reversible bronchoconstriction that was dose-dependent. Flax dust appeared to contain a more effective bronchoconstrictor agent than cotton dust. The effects of flax dust were not affected by atropine, mepyramine or methysergide. However, flax-induced bronchoconstriction was antagonized by FPL-55712, nordibydroguaiaretic acid and cromoglycate. Cotton dust was similarly affected by these pharmacological agents. The results suggest that in this lung preparation flax and cotton dusts may induce bronchoconstriction by releasing leukotrienes.