The aim of this study was the evaluation of the in vitro production of prostacyclin, and of tissue plasminogen activator (tPA) and its inhibitor, PAI-1, by human endothelial cells cultured in the presence of polyethylene terephthalate (PET). After a 48 h contact between the cells and the polymer, the concentration of 6-keto-PGF1 alpha, a stable metabolite of prostacyclin, tPA, and PAI-1, was assayed on the supernatants. Contact of the endothelial cells with PET produced a highly significant reduction of 6-keto-PGF1 alpha with respect to control cultures. Tissue plasminogen activator concentration in the supernatants of the cultures in contact with the material was similar to that observed in the controls, while PAI-1 production was significantly reduced. It can be concluded that the contact between endothelial cells and PET determines a reduction in the platelet aggregability and an increase of the fibrinolytic activity due to a decrease in PAI-1, while tPA concentration remains unchanged.