Field-dried cotton bract, a contaminant of cotton dust, has been implicated in byssinosis pathogenesis. The results from this study demonstrated that a standard bract extract (FDBE) could be prepared. FDBE was antigenic in rabbits; however, precipitating antibodies were not detectable in the serum of cotton textile mill workers. Although exposure of mononuclear leukocytes (MNL) to FDBE did not alter intracellular cyclic AMP levels, it did induce changes in the cyclic AMP response of MNL to isoproterenol and prostaglandin E1. These changes were FDBE dose-dependent. The possible relationship of these findings to pathogenesis of byssinosis is discussed.