Curcumin, a major component of turmeric, has been shown to exhibit anti-oxidant and anti-inflammatory activities. The present study was performed to determine whether curcumin is efficacious against both collagen-induced arthritis (CIA) in mice and IL-1beta-induced activation in fibroblast-like synoviocytes (FLSs). DBA/1 mice were immunized with bovine type II collagen (CII) and treated with curcumin every other day for 2weeks after the initial immunization. For arthritis, we evaluated the incidence of disease and used an arthritis index based on paw thickness. In vitro proliferation of CII- or concanavalin A-induced splenic T cells was examined using IFN-gamma production. Pro-inflammatory cytokines TNF-alpha and IL-1beta were examined in the mouse ankle joint and serum IgG1 and IgG2a isotypes were analyzed. The expression levels of prostaglandin E(2) (PGE(2)), cyclooxygenase-2 (COX-2), and matrix metalloproteinases (MMPs) in human FLSs were also determined. The results showed that compared with untreated CIA mice, curcumin-treated mice downregulated clinical arthritis score, the proliferation of splenic T cells, expression levels of TNF-alpha and IL-1beta in the ankle joint, and expression levels of IgG2a in serum. Additionally, by altering nuclear factor (NF)-kappaB transcription activity in FLSs, curcumin inhibited PGE(2) production, COX-2 expression, and MMP secretion. These results suggest that curcumin can effectively suppress inflammatory response by inhibiting pro-inflammatory mediators and regulating humoral and cellular immune responses.