Mast cell (MC) activation in the rheumatoid lesion provides numerous mediators that contribute to inflammatory and degradative processes, especially at sites of cartilage erosion. MC activation in rheumatoid synovial tissue has often been associated with tumour necrosis factor (TNF)-α and interleukin (IL)-1β production by adjacent cell types. By contrast, our in situ and in vitro studies have shown that the production of IL-15 was independent of MC activation, and was not related to TNF-α and IL-1β expression. Primary cultures of dissociated rheumatoid synovial cells produced all three proinflammatory cytokines, with production of IL-1β exceeding that of TNF-α, which in turn exceeded that of IL-15. In vitro cultures of synovial macrophages, synovial fibroblasts and articular chondrocytes all produced detectable amounts of free IL-15, macrophages being the most effective.