Bacteria and bacterial antigens strongly induce cytokine secretion by peripheral blood leukocytes and thereby initiate an inflammatory cascade with potentially deleterious consequences for the host. The present study focussed on receptors and signal transduction pathways involved in activation of interleukin (IL)-18 by heat-inactivated Gram-positive Staphylococcus aureus Cowan strain I (SAC). Similarly to IL-12/IL-12p40, IL-10 and IFN-gamma, SAC dose-dependently activated IL-18. Secretion of IL-18 was independent of functional activity of IL-10, IL-12 or IFN-gamma. Lipoteichoic acid (LTA), a structural component of SAC, was not sufficient for activation of IL-18, while it dose-dependently induced IL-10. In contrast to IL-12, blockade of CD14 only partially diminished secretion of IL-18 and did not affect secretion of IL-10, suggesting involvement of other receptors (e.g., Toll-like receptors) in SAC responses. Further down-stream however, secretion of IL-10, IL-12 and IL-18 was uniformly inhibited by blockade of G-protein-mediated kinase activation by mastoparan. Secretion of IL-18 required phosphatidylinositol-3'-kinase, and secretion of IL-12 phosphotyrosine kinase activity. The data demonstrate that SAC potently activates secretion of IL-18 by peripheral blood mononuclear cells with differential involvement of cell-surface receptors and signal transduction pathways as compared to other natural killer- and T cell-promoting cytokines.