The monocolonal antibody 314G8 reacts with human mucosal addressin cell adhesion molecules-1 (MAdCAM-1), a key player in mediating the infiltration of leukocytes into chronically inflamed tissue. MAdCAM-1 is a cell-surface Ig superfamily member composed of two extracellular Ig domains, followed by a mucin-like domain, a transmembrane domain and a short cytoplasmatic domain. It interacts via its N-terminal Ig domain with the lymphocyte homing receptor alpha4beta7, which plays a critical role in forming the gut-associated lymphoid system. MAdCAM-1 promotes the adhesion of T- and B cells, monocytes/macrophages, and potentially eosinophils, basophils, and differentiated mast cells to the vascular endothelium. Mucosal addressin cell adhesion molecule-1 RNA transcripts are predominantly expressed in the small intestine, mesenteric lymph nodes, colon and spleen; and are very weakly expresssed in human pancreas and brain. The monocolonal antibody 314G8 recognizes a site in the N-terminal Ig domain of MAdCAM-1. The monoclonal antibody 314G8 detects MAdCAM-1 on venules in the spleen and small intestine. MAdCAM-1 is strongly expressed in the synovium of osteoarthritis patients, predominantly on the endothelial lining of blood vessels, but also within the vessel lumen. The monoclonal antibody 314G8 may be useful in diagnosis of inflammation in humans by monitoring the presence and levels of MAdCAM-1.