Tenascin is a major extracellular glycoprotein known to have important functions in processes such as wound repair and embryogenesis including bone and cartilage formation. The expression of this molecule in articular cartilage and synovium from normal and abnormal (OA and inflammatory joint disease including RA) human knee joints was studied by an immunohistological technique using paraffin embedded tissue and a specific anti-human tenascin monoclonal antibody (BC4). The results show that in normal articular cartilage tenascin is expressed in small amounts in the surface zone and in synovium is present in significant levels in the walls of blood vessels only. In diseased joints expression is greatly increased both in articular cartilage and synovium. Increased production of tenascin is likely to be part of a reparative response in injured joints the understanding of which may suggest novel mechanisms to modify disease progression in degenerative and inflammatory joint disease.