Cotton dust was treated with ethanol containing either acid or alkali to remove contaminants. Aqueous extracts of these chemically treated dusts were examined in three pharmacological tests using guinea pig tissue. Extracts of the standard untreated dust were effective in all the tests employed:- bronchoconstriction of the perfused isolated lung, contraction of the isolated tracheal spiral and tritium release from (3)H-arachidonic acid-loaded alveolar macrophages. In general, the extracts of the chemically treated dusts were less active than the extract from the control sample. The most effective treatment for removal of activity from cotton dust was a five-times extraction procedure with sodium hydroxide in ethanol.