Factor VIII/von Willebrand factor (vWF) was sought by immunofluorescence in or on canine platelets and blood vessels. None was found on normal canine platelets and little was present in normal canine arteries, veins and capillaries compared with normal human blood vessels. However, free granules of vWF were scattered in platelet-rich canine plasma and occasional granules appeared on small clumps of platelets when ristocetin or collagen was added to the plasma. When the same platelets were suspended in human plasma and restocetin or collagen was added, more clumps were formed and more vWF (human) was associated with these clumps. When thrombin was added to canine platelets in either canine or human serum, more solid, small clumps of platelets were formed and stained with the anti-vWF sera. When thrombin was added to canine platelets in either canine or human plasma, a single large clot was formed which stained brightly for vWF.
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