Platelets suspended in serum or afibrinogenemia plasma (heated or congenital) adhered to glass slides previously exposed to normal plasma or to fibrinogen but not to slides exposed to serum or afibrinogenemic plasma. Platelets in platelet-rich plasma from a patient with congenital afibrinogenemia adhered to glass coated with plasma from a normal subject or von Willebrand patient but not to areas coated with afibrinogenemic plasma or serum. Platelets in platelet-rich plasma from two thrombasthenia patients failed to adhere even when the slide was coated with fibrinogen or plasma. At times, a much higher percentage of platelets adhered to areas exposed to plasma for 5 sec than to areas exposed for 3 min. According to other evidence, the protein film adsorbed from plasma onto a surface no longer reacts with antiserum to fibrinogen after 3 min. We conclude that a fibrinogen layer must first be adsorbed to glass for the platelets to adhere.