Cytokine levels are increased in the synovial fluid of affected joints from patients with inflammatory joint diseases. The aim of our study was therefore to determine if and to what extent immmunologically defined subpopulations of mononuclear cells (MNC) in the synovial fluid are responsible for the increased levels of interleukin-1beta (IL-1beta) and interleukin-6 (IL-6) in affected joints. Lipopolysaccharide (LPS) stimulated peripheral MNC were used as positive controls. While soluble IL-1beta (median 167 pg/ml) and IL-6 (median 508 pg/ml) levels were significantly elevated in the synovial fluids tested, IL-1beta and IL-6 were demonstrated by flow cytometry in only a small subpopulation (<=11%) of mononuclear synovial fluid cells in 7/13 patients. Our results suggest that elevated IL-1beta and IL-6 levels in the synovial fluid of inflammatory joints are derived mainly from cells in the synovial membrane and only to a minor extent from cells in the synovial fluid itself.