Cotton bract has been proposed as a probable byssinogenic agent. Mill workers are exposed to bract which has weathered in the field, but it is not known whether biologic effects of bract are due to intrinsic plant compound(s) or to contamination occurring during field weathering. Exposure of mononuclear leukocytes (MNL) to green bract extract (GBE) lowered basal cyclic AMP levels. Concomitant exposure of MNL to GBE plus agonist altered MNL cyclic AMP levels in a dose-dependent fashion. Differences between immunologic and pharmacologic test results of GBE and field-dried bract extract were mainly quantitative, suggesting the bioactive agent(s) in cotton bract is intrinsic to the plant.