Primary synovial cells with high passage number demonstrate increased production of proinflammatory mediators in response to inflammatory stimuli compared with cells with low passage number. This study used synovial cells to learn how different numbers of serial subculture passages affect the production of proinflammatory mediators in response to interleukin (IL)-1β. Synovial cells were serially subcultured in flasks until passage 7. During cell passage, synovial cells were treated with IL-1β for 24 h. Levels of proinflammatory mediators were analyzed by ELISA, real-time PCR, and Western blot. Synovial cells at passage 7 had elongated morphology and higher activity of β-galactosidase, a marker of senescence, than those at passage 3. Production of IL-6, IL-8, vascular endothelial growth factor (VEGF), and prostaglandin E2 (PGE(2)) in response to IL-1β was significantly increased in cells at passage 7 compared with passage 3. To evaluate the mechanism of this different response, IL-1 receptor expression was studied in cells stimulated with IL-1β. Compared with cells at passage 3, cells at passage 7 had stronger expression of IL-1 receptors 1 and 2, as well as significantly increased expression of both receptors, in response to IL-1β. This finding suggests that differential production in response to inflammatory stimuli associated with cell passages should be considered when in vitro experiments are used to evaluate the production levels of proinflammatory mediators.