Dendritic cells have been isolated from peripheral blood and inflamed synovial tissue and synovial fluid of patients with rheumatoid arthritis and from normal peripheral blood. Synovial and blood dendritic cells are strongly positive for CD45 and MHC class II antigens, and lack almost all other mononuclear cell markers. Thus, in most respects they have the same characteristics as lymphoid dendritic cells in mice. Synovial and blood dendritic cells are very potent accessory cells for T lymphocyte responses, and much more effective than monocytes. Synovial dendritic cells also spontaneously produce interleukin 1. The accessory function is inhibited by an antibody to interleukin 1. Synovial dendritic cells may thus be critical for starting and perpetuating the chronic inflammation seen in rheumatoid arthritis.