Platelet adhesion to glass has been directly determined with a cover slip chamber. These studies have separated the functions of platelet adhesion to a foreign surface from platelet cohesion (aggregation). The forces of the platelet-glass interaction have been studied, and the qualitative variables affecting this interaction have been defined. Fibrinogen and calcium or magnesium are required for adhesion of either unwashed or washed platelets in plasma. Platelet adhesion in fresh serum requires thrombin and probably the fibrinogen of the platelet surface, since washed platelets are not adherent in fresh serum. Platelets incubated at 37°C for 48 hr can neither spread nor adhere to glass, a defect that may reflect decreased platelet surface area and membrane deformability. Platelets are equally as adherent to siliconized glass as to untreated glass surfaces. However, platelets cannot adhere to glass treated with the hydrogen-bonding polymer, poly(ethylene oxide). In contrast to silicone, poly(ethylene oxide)-treated glass surfaces do not impede blood coagulation. These surface properties further distinguish the two major factors involved in thrombogenesis: platelet adhesion and plasma coagulation.