Interleukin 1 (IL-1) has been implicated as an inflammatory mediator in rheumatoid arthritis (RA). Many cell types, including macrophages, lymphocytes, fibroblasts, and endothelial cells, can produce IL-1 and it is known that IL-1 production is under transcriptional control. It has, however, been difficult to define in vivo the predominant cellular source of this mediator in RA. Here, we have used the combination of in situ hybridization of mRNA and cellular immunophenotyping with monoclonal antibodies to show that the IL-1 beta gene is expressed predominantly by CD14-positive macrophages in synovial tissue from patients with RA. Synovial macrophages were also associated with the immunoreactive IL-1 peptide. These cells appear to be the major source of IL-1 beta within the rheumatoid synovium in vivo and must be regarded as playing a central role in the chronic inflammation and joint destruction of RA.