IL-6, an important mediator of the acute phase response, has been implicated in the pathogenesis of rheumatoid arthritis (RA). Many cell types including macrophages, T cells, B cells, endothelial cells and fibroblasts can produce this cytokine and production is largely regulated at the level of gene transcription or mRNA stabilization. In this paper we have first measured the levels of IL-6 activity in synovial fluid (SF) and serum from patients with RA and then localized IL-6-producing cells in the synovium by in situ hybridization combined with immunophenotyping. Patients with RA had raised levels of IL-6 in both SF and serum compared with patients with osteoarthritis and age-matched healthy controls. In individual RA patients tested serially after admission to hospital, serum IL-6 was initially raised and, unexpectedly, increased with clinical improvement. In situ hybridization of IL-6 mRNA showed positive cells both in the lymphocyte-rich aggregates and adjacent to small blood vessels. With immunophenotyping it was found that cells containing IL-6 mRNA were often in contact with CD14+ tissue macrophages and double immunophenotyping revealed that immunoreactive IL-6 was often associated with synovial T cells.