Interleukin 12 (IL-12) and IL-18 act synergistically to stimulate interferon gamma (IFN-γ) production; moreover, IL-1 and tumor necrosis factor (TNF) may also augment IFN-γ synthesis. We have investigated the relative contributions of these cytokines in the production of IFN-γ and TNF by the Gram-positive bacterium Staphylococcus epidermidis, using the specific cytokine inhibitors IL-18 binding protein (IL-18BP), IL-1 receptor antagonist (IL-1Ra), anti-IL-12 antibodies (anti-IL-12 Ab), and TNF binding protein. Inhibition of caspase-1 reduced IFN-γ and IL-1β levels (by 80 and 67%, respectively) when heat-killed S. epidermidis was added to whole human blood cultures. IL-18BP reduced S. epidermidis-induced IFN-γ (77% maximal suppression). In contrast, blocking IL-1 receptors by IL-1Ra had no effect on IFN-γ production. Blocking endogenous IL-12 and TNF reduced IFN-γ production by 69 and 36%. S. epidermidis-induced TNF-α was inhibited by IL-18BP and IL-1Ra, but not anti-IL-12 Ab, whereas IL-8 production was unaffected by any of the specific cytokine blocking agents. In conclusion, S. epidermidis stimulates IFN-γ which is IL-18, IL-12 and TNF-dependent, but IL-1 independent.