Our previous study established protein gene product 9.5 (PGP 9.5), a ubiquitin C-terminal hydrolase, as a specific cytochemical marker of synovial lining cells (type B synoviocytes) in the horse joint. The present study aimed to detect PGP 9.5 in the synovial fluid and shows that PGP 9.5 is a valuable marker of osteoarthritis in the horse. Immunohistochemical staining confirmed rich and consistent localization of PGP 9.5 immunoreactivity in the cytoplasm of synovial lining cells in the normal horse joint. Western blot analysis of synovial fluid from normal joints could detect a significant band corresponding to that contained in the brain and synovial membrane extracts. When 60 synovial fluid samples from normal and abnormal joints were assayed with an enzyme-linked immunosorbent assay (ELISA) system, the concentration of PGP 9.5 tended to be elevated in osteochondrosis dissecance, inflammatory arthropathy and intra-articular fracture, among which a statistiture and the control. Thus, this study demonstrated the possibility that PGP 9.5, derived from synovial lining cells, may be a new biochemical marker for arthritic disorders of the horse.