Matrix metalloproteinases (MMPs) are considered important mediators of tissue damage in joint diseases. The levels of MMPs 2 and 9 were measured in samples of synovial fluid from 20 joints in seven dogs with rheumatoid arthritis by gelatin zymography. The results were compared with the actual gelatinolytic activity of the fluid measured in a gelatin-degradation ELISA. The gelatinolytic activity in synovial fluid from arthritic joints was markedly greater than that in fluid from disease-free joints. The zymographic activity attributable to MMP-9 (identified by Western blotting) was absent from synovial fluid from control joints but prominent in fluid from arthritic joints, and in these joints the presence of a 75 kDa form of MMP-9 was correlated with the gelatinolytic activity of the fluid measured by the ELISA (r=0.81, P<0.05). Synovial fluid from one dog with rheumatoid arthritis was examined before and after treatment with corticosteroids. After treatment its zymographic pattern had returned to normal.