Interleukin-18 (IL-18) is a novel cytokine with interferon-gamma (IFN-gamma)-inducing activity, thus favoring the T-helper type 1 (Th-1) pathway. The present study attempts to define the role of IL-18 on the functions of lymphocytes isolated from malignant pleural effusions (EAL, effusion-associated lymphocytes). EAL from 10 patients with malignant pleural effusion were incubated with IL-2, IL-12, or IL-18 with/without a alpha CD3 antibody. ELISA, proliferation, and cytotoxicity assays were performed. IL-18 alone was found to have no significant effect on EAL in terms of cytokine production, lymphocyte proliferation, or cytotoxicity against tumor targets. IL-18 also had no significant additive or synergistic effect on IL-2, IL-12, or alpha CD3 co-cultured EAL. However, when IL-18 was used with IL-12, the highest IFN-gamma/IL-10 ratio was derived, suggesting that these two cytokines had an additive effect in leading EAL from the Th-2 to the Th-1 pathway. Furthermore, 1 of 5 patients' EAL had its strongest cytolytic activity against an autologous tumor when the EAL was cultured with IL-2 plus IL-18, as compared with the other 4 patients whose EAL cytolytic activity against autologous tumor was highest when using IL-2 plus alpha CD3. These findings suggest that IL-18 alone did not have a significant effect on EAL, and that IL-18 did not enhance alpha CD3's activity on EAL. However, its additive effect with IL-12 in the Th-1 pathway and with IL-2 in its cytolytic activity against an autologous tumor deserve further studies.