Our pre-clinical studies have demonstrated a pathogenic role for TNF alpha in RA. Firstly, TNF alpha and its receptors are upregulated and co-expressed in the synovium and cartilage-pannus junction of RA joints. Secondly, mononuclear cells from RA joints maintained in culture produce many cytokines with pro-inflammatory activity, including TNF alpha. Neutralizing TNF alpha antibodies in vitro reduces the production of these pro-inflammatory cytokines, including IL-1, IL-8, and GM-CSF. Thirdly, when injected into arthritic DBA/l mice with collagen-induced arthritis, monoclonal anti-TNF antibodies decrease inflammatory damage of joints. Clinical trials employing cA2, a monoclonal chimeric anti-TNF alpha antibody, in open-label and randomized placebo-controlled studies have demonstrated a dose-dependent efficacy with impressive improvement in disease activity and acute phase responses lasting several weeks. We conclude that TNF alpha is a critical mediator of inflammation in RA and is an important therapeutic target in this disease.