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von Willebrand factor (VWF) is crucial to hemostasis, but also plays a role in inflammatory processes. Unfortunately, no proper monoclonal antibodies to study VWF function in mice are currently available. We therefore aimed to generate single-domain antibodies (sdAbs) recognizing murine VWF and blocking its function in vivo.Llama-derived sdAbs recognizing both human and murine VWF were isolated via phage display technology. One of them (designated KB-VWF-006) recognized the VWF A1 domain with picomolar affinity. This sdAb avidity was strongly enhanced via dimerization using a triple Ala linker (KB-VWF-006bi). When administered in vivo to wild-type mice, KB-VWF-006bi dose dependently induced bleeding in a tail clip model. In 2 distinct models of inflammation, KB-VWF-006bi efficiently interfered with leukocyte recruitment and vascular leakage.KB-VWF-006bi is an sdAb recognizing the A1 domain of human VWF and murine VWF that interferes with VWF–platelet interactions in vivo. By using this sdAb, we now also show that the A1 domain is pertinent to the participation of VWF in the inflammatory response.