Vascular endothelial cells (EC) are non-thrombogenic. The basis for this property is still essentially unknown. This paper summarizes our previous studies on EC associated glycosaminoglycans and on the specific binding of heparin to EC and describes the preventive effect of heparin sulphate (HS) on platelet adhesion to collagen; attempts to affect the binding of platelets to EC by specific removal of endogenous HS were, however, unsuccessful. The possible importance of EC surface HS for interactions with coagulation factors is discussed.