Platelet adhesion to subendothelium in the absence of VWF or functional GPIbα is decreased. Venules of approximately 200- to 300-μm in diameter were visualized in the mesentery of live mice after ferric-chloride injury. Numbers of single fluorescent platelets transiently tethering and/or stably adhering to vessel wall within 2 to 3 minutes after ferric-chloride injury were determined. (A) Significantly fewer platelets adhered in VWF^−/− venules compared with WT. The infusion of r-hu-FVIII in VWF^−/− mice did not increase platelet adhesion. (B) Fluorescent IL4Rα/GPIbα-tg or WT platelets were infused in WT mice. The adhesion of IL4Rα/GPIbα-tg fluorescent platelets to injured venules was significantly less than that of WT platelets. Error bars indicate SEM.