In this study, we investigated interleukin (IL)-18 and IL-12 following in vitro stimulation with either the 30-kDa or purified protein derivative (PPD) antigens (Ag) of pleural mononuclear cells from 12 cases of tubercular pleurisy (TB-PMC) and 8 cases of malignant pleurisy (MG-PMC). Ag-stimulated TB-PMC produced significantly more IL-12 than did MG-PMC and the levels correlated with those of IFN-${gamma}$. Although elevated IL-18 levels were found in freshly isolated pleural fluids, in vitro IL-18 production in response to either Ag was dramatically decreased in TB-PMC. Pro-IL-18 mRNA was detected before and after Ag stimulation in TB patients. Supernatants from the Ag-stimulated TB-PMC significantly suppressed IL-18 production in normal peripheral blood mononuclear cells (PBMC) and primary malignant cells over an 18 h incubation period. In addition, this suppressive activity was not inactivated by either heat or trypsin. Our findings imply that modulation of IL-12 and IL-18 levels may contribute to the Th1 elevation induced in human TB-PMC by the 30-kDa and PPD antigens.