Tannins comprise a significant portion of cotton bracts, the component of dust implicated in the pathogenesis of byssinosis. We have previously shown that these tannins cause toxicity to vascular endothelial cells (EC) and are potent platelet agonists. Here we report the effect of tannin exposure on subsequent binding of washed porcine platelets to cultured porcine EC derived from the thoracic aorta and pulmonary artery. The EC were exposed for 15 min to tannin dissolved in serum-free medium. The monolayers were then incubated with washed, (51)Cr-labeled porcine platelets for 30 min on a rotary shaker. Tannin caused a significant, dose-dependent increase of platelet adherence to both endothelia; aortic adherence was higher than that to pulmonary artery cells. Platelet binding was localized to intercellular junctions, the specific region that we have previously shown to exhibit a morphological change following exposure to tannin.