Dendritic cells (DC) from the synovial inflammatory tissue and peripheral blood of patients with rheumatoid arthritis and from the peripheral blood of normal blood donors were compared with the autologous monocytes for their capacity to produce and release interleukin 1 (IL-1). Synovial DC often spontaneously released higher amounts of IL-1 activity than unstimulated and lipopolysaccharide-stimulated peripheral blood DC and monocytes. The IL-1 production by both DC and monocytes increased after stimulation with bacterial lipopolysaccharide. In contrast with synovial DC the peripheral blood DC from both patients with rheumatoid arthritis and normal controls released less IL-1 activity than peripheral blood monocytes did. Inhibition with an antiserum to IL-1 revealed that IL-1 production is important for the accessory activity of the peripheral blood DC. Thus human DC from inflammatory sites and peripheral blood produce IL-1 activity.