We measured and compared the activities of various kinds of proteinases, such as cysteine, serine, aspartic, and metalloproteinases, in synovial fluids of 16 patients with rheumatoid arthritis (RA) and 18 patients with osteoarthritis (OA). More than 19-fold higher activity of cathepsin B and about 6-fold higher activity of prolylendopeptidase, compared to those of OA, were accumulated in RA fluid. Moreover, levels of cathepsins B and S using the corresponding sandwich enzyme immunoassays were statistically higher in RA fluid than those in OA. Significant amounts of 41-kDa and 35-kDa procathepsin L were detected in RA fluid using gelatin zymography, while 41-kDa enzyme alone was detected in OA. Cathepsin B in RA fluid could degrade collagen, and this degradation was suppressed by the addition of CA-074, a specific inhibitor of cathepsin B. Therefore, cathepsin B may participate in joint destruction of RA, and its inhibitor may be effective for RA care.