Vascular adhesion protein-1 (VAP-1) is an endothelial molecule that possesses both adhesive and enzymatic properties in vitro. So far, however, elucidation of its in vivo function has suffered from the lack of function-blocking reagents that are suitable for use in animal models. In this work we produced monoclonal antibodies against murine VAP-1 and characterized them using in vitro binding assays. We then examined whether the antibodies could prevent leukocyte migration in in vivo inflammation models, including two acute models (peritonitis induced with proteose peptone and interleukin-1 and air pouch inflammation enhanced by CCL21) and one chronic model (autoimmune diabetes in nonobese diabetic mice). Antibodies 7-88 and 7-106 inhibited migration of granulocytes and monocytes in both acute models of inflammation. Strikingly, antibody 7-88 significantly prevented diabetes in a subset of nonobese diabetic mice. The results show for the first time that in mouse models of inflammation, VAP-1 mediates leukocyte trafficking to sites of inflammation and thus is a potential target for anti-inflammatory therapies.