Extracts of cotton dust and bract induced in vitro thromboxane release from human platelets. Thromboxane release was both time and dose dependent. Cotton green bract extract (CGBE) was significantly more potent than either cotton dust extract (CDE) or cotton brown bract extract (CBBE) while CBBE was least active. Thromboxane release was inhibited by both indomethacin and 7-(1-imidazolyl) heptanoic acid (7-IHA). These studies show for the first time a possible relationship between thrombocytopenia noted in cotton mill workers and effects on human platelets, in particular, the in vitro release of thromboxane, a potent respiratory smooth muscle contractor. In vivo platelet thromboxane release may prove to be an important mechanism in the pathogenesis of byssinosis.