We assayed the activities of collagenase and monoamine-oxidase (MAO) in the fluid of the patients with rheumatoid arthritis (RA) and examined the relationship between collagenase, MAO activities and the grade of joint inflammation. The mean collagenase value of RA patients was significantly higher than that of osteoarthritis (OA) patients. A significant positive correlation was present between collagenase activity, leukocyte counts, and beta-glucuronidase activity in RA synovial fluid. These indices are considered to be parameters of the grade of joint inflammation. There was no correlation between collagenase activity, erythrocyte sedimentation rate (ESR), gamma-globulin level, C-reactive protein (CRP), rheumatoid factor, acid-phosphatase activity and the stage of RA (grade of joint destruction). The mean MAO value of RA synovial fluid was significantly higher than that of OA synovial fluid. The mean activity of the group of patients whose CRP values were below two positive was significantly higher than that of the group of patients whose CRP values were over two positive. The mean MAO activity in stage 3 or 4 group was significantly higher than that in stage 1 or 2. These findings suggest that collagenase activity is intimately correlated with the grade of local Inflammation in RA and that the MAO activity is closely correlated with the grade of joint fibrosis.