Our objective was to clarify the DNA-binding activity of transcription factor NF-kappa B as related to cytokine induction in the synovium of patients with rheumatoid arthritis (RA) and osteoarthritis (OA). Synovial membranes were obtained during arthroplasty of the knee from 7 patients with RA and 4 patients with OA. Nuclear extracts obtained from the synovial membranes were examined by electrophoretic mobility shift assay to determine the DNA-binding activity of NF-kappa B. Markedly high DNA-binding activity of NF-kappa B was detected in the synovial membranes of RA patients, while virtually no activity was observed in those of OA patients. These results suggest that the high induction of NF-kappa B in the nuclear extracts may be relatively specific to synovial membranes in RA. NF-kappa B may regulate the production of cytokines at the site of synovial inflammation.