The involvement of IL-6 in the pathogenesis of rheumatoid arthritis (RA) has been recently demonstrated. In the present study, we investigated the cellular and molecular mechanisms involved in the spontaneous IL-6 production by the fibroblast-like synoviocytes (FLSs) obtained from patients with RA. Cloned FLSs were established from the bulk cultures of FLSs by the limiting dilution method. Some FLS clones spontaneously produced large amounts of IL-6, whereas others produced low amounts of it. Neither anti-human TNF-alpha nor IL-1 antibody affected spontaneous IL-6 production of these FLS clones, suggesting that IL-6 production of the FLSs was endogenously up-regulated. A luciferase reporter plasmid containing the human IL-6 promoter region was significantly transcribed when transfected into the IL-6 high-producing clones, indicating that the rheumatoid FLSs retained constitutive transcriptional activity of the IL-6 gene. Electrophoretic mobility shift assays revealed that the binding activity of p50 and p65 NF-kappaB subunits and CBF1 was significantly enhanced in the IL-6 high-producing clones compared with that of IL-6 low-producing clones and cultured sarcoma cells, suggesting that spontaneous activation of NF-kappaB and CBF1 may lead to the constitutive transcription of the IL-6 gene by rheumatoid FLSs.