Chelidonium majus L. has multiple applications in Korean traditional medicine because of its anti-tumoral, cytotoxic, anti-inflammatory and anti-microbial activities and has long been known to have anti-inflammatory effects. However, no study on the anti-arthritic activity of Chelidonium majus has been reported in vivo. Rheumatoid arthritis (RA) is a systemic autoimmune disease with chronic inflammation characterized by hyperplasia of synovial cells in affected joints, which ultimately leads to the destruction of cartilage and bone. Cytokine production and gene expression were assessed during CIA (collagen-induced arthritis) model mice in knee joint, lymph node (LN), and spleen, using ELISA and competitive RT-PCR. DBA/1J mice were immunized with bovine type II collagen. After a second collagen immunization, mice were treated with CME orally at 400, 40mg/kg once a day for 4 weeks. The severity of arthritis within the knee joints was evaluated by histological assessment of cartilage destruction and pannus formation. Administration of CME significantly suppressed the progression of CIA and inhibited the production of TNF-alpha and IL-6 in spleen and lymph node. The erosion of cartilage was dramatically reduced in mouse knees after treatment with CME. In conclusion, our results demonstrates that CME significantly suppressed the progression of CIA and that this action was characterized by the decreased production of TNF-alpha, IL-6, IFN-gamma, B cells, gammadelta T cells (in spleen) and increased proportion of CD4+CD25+ regulatory T cells in vivo. In the serum of CME-treated mice, the levels of IgG and IgM RA factor were decreased.