Interleukin (IL)-12 enhances natural killer (NK) activity and induces interferon gamma (IFN-gamma) production. Recently, it was shown that IL-12 induces IL-10 production by human T cells and NK cells, as a negative feedback for IL-12-induced immune responses. In the present study, in order to investigate the functions of host immune cells existing in contact with cancer cells, we examined the effect of IL-12 on the induction of non-major histocompatibility complex (MHC)-restricted killer activity and of IFN-gamma and IL-10 production by pleural and peripheral blood mononuclear cells (MNC), isolated from 40 lung cancer patients and 20 control subjects. IL-12 induced significant killer activity in pleural MNC from lung cancer patients, as well as those in peripheral blood, against a small cell lung cancer cell line (SBC-3). In lung cancer patients, pleural MNC incubated with IL-12 produced more IFN-gamma than blood MNC. In addition, when stimulated with both IL-12 and IL-2, pleural MNC produced more IL-10 than blood MNC. This is the first study reporting that MNC from pleural effusions of patients with lung cancer can produce both type 1 (IFN-gamma) and type 2 (IL-10) cytokines following exposure to IL-2 and IL-12. These observations suggest that control of IL-10 production at the microenvironment level may be important for the efficacy of human lung cancer immunotherapy with IL-12.