KE-298 is a novel antirheumatic drug which suppresses various animal models of arthritis by inhibiting the production of inflammatory cytokines. In a phase II study of rheumatoid arthritis patients, ingestion of KE-298 led to significant improvements in the Lansbury index. The objective of the present study was to clarify the effects of KE-298 against synovium functions, using rheumatoid arthritis synoviocytes. We investigated the effects of KE-298 on the production of matrix metalloproteinases and tissue inhibitor-1 of metalloproteinases and bone absorptive mediators including interleukin (IL)-6 and prostaglandin (PG) E2 in tumor necrosis factor (TNF)-alpha-stimulated rheumatoid arthritis synoviocytes. Rheumatoid arthritis synoviocytes were obtained from knee joints of rheumatoid arthritis patients and type B fibroblast-like synoviocytes were stimulated with TNF-alpha, with or without KE-298. The contents of metalloproteinases and tissue inhibitor-1 of metalloproteinases and IL-6 and PGE2 in culture media were measured by enzyme-linked immunosorbent assay. KE-298 significantly suppressed TNF-alpha-induced production of promatrix metalloproteinase-1 and IL-6, in a dose-dependent manner, but not that of tissue inhibitor-1 of metalloproteinases. The potential of KE-298 to suppress the production of matrix metalloproteinase-1 and IL-6 may explain its efficacy on rheumatoid arthritis.