Lysosomal enzymes were examined in synovial fluid, synovium, synovial fluid leukocytes and peripheral blood leukocytes of 40 rheumatoid arthritis (RA) patients and in synovial fluid and synovium of 19 osteoarthritis (OA) patients. Complement level and anticomplementary activity were examined in 13 RA synovial fluid. The results are summarized as follows: 1, In the supernatant synovial fluid and the synovium of RA patients, cathepsin D (C-D) and β-glucuronidase (β-G) activities were higher than those of OA patients. A positive correlation was present between the synovial fluid leukocyte count and C-D activity and between synovial fluid leukocyte count and β-G activity. RA synovial localization of β-G activity was demonstrated by light microscopy. The synovial lining cell of RA patients showed the strongest β-G localization. 2, No significant differences were found in C-D and β-G activities between RA synovial fluid leukocytes, RA peripheral blood leukocytes and normal human peripheral blood leukocytes. These results suggested that there were little differences in the inflammatory leukocytes and the noninflamatory leukocytes. 3, C-D and β-G activities in the supernatant synovial fluid and synovial fluid leukocytes were higher in RA patients with positive RF in synovial fluid than negative RF in synovial fluid. The results of this study suggested that the RF was displaced to the inflamation RA joint. 4, The presence of low complement levels and anticomplementary activity in RA synovial fluid suggested the role of the immune complex in the RA synovial cavity. 5, These results suggest that the high level of lysosomal enzymes in RA synovial fluid was due chiefly to the synovial membrane.