Human cathepsin G (EC 3.4.21.20) has been reported to have the in vitro chemotactic activity for human monocytes. In this study, we examined the role of cathepsin G in monocyte involvement in joint inflammation of rheumatoid arthritis (RA) as a monocyte chemoattractant. Eighteen patients with RA and four patients with osteoarthritis (OA) were used in this study. Thiobenzylester substrate, Succ-Phe-Leu-Phe-S-Bzl, was used to measure the activity of cathepsin G in synovial fluids. Monocyte migration induced by cathepsin G and synovial fluids was assessed by a 48-well microchemotaxis chamber technique. Immunohistochemical staining was performed to determine the cellular origin of cathepsin G in RA synovial tissue. A very low activity of cathepsin G was detected in synovial fluids from patients with OA. On the other hand, significantly increased activity of cathepsin G was detected in patients with RA when compared with the value of OA patients. A considerable monocyte chemotactic activity was detected in the synovial fluid of RA patients, and the activity was partially decreased by the treatment with inhibitors for cathepsin G, alpha1-antichymotrypsin and phenylmethylsulfonyl fluoride. The activity of cathepsin G was significantly correlated with the neutrophil counts in synovial fluids and the concentration of interleukin-6. Immunohistochemical studies showed that cathepsin G was strongly expressed by synovial lining cells, and weakly expressed by macrophages and neutrophils in synovial tissues. This study indicates that the monocyte chemotactic activity of cathepsin G may have a role in the pathogenesis of RA synovial inflammation.