Interleukin (IL) 15 is a novel cytokine with IL-2-like activity. In this study, we examined the effect of IL-15 on induction of non major histocompatibility complex (MHC)-restricted killer activity and of type 2 cytokine production by peripheral blood and pleural mononuclear cells (MNCs), from 34 lung cancer patients and 20 control subjects. IL-15 induced significant killer activity in blood MNCs from lung cancer patients as well as control subjects against a small-cell lung cancer cell line (SBC-3). Effective killer induction by IL-15 was observed even in blood MNCs and pleural MNCs from the site of tumour growth in advanced lung cancer patients. IL-12 had an additive effect with a suboptimal dose of IL-15 in induction of killer activity. In the case of MNCs from lung cancer patients, IL-10 production was more prominent when cells were incubated with IL-2 than with IL-15. IL-5 production was observed in MNCs from lung cancer patients stimulated with IL-2, but not with IL-15. These observations suggest that IL-15, by virtue of its lesser induction of type 2 cytokine, may be a better candidate than IL-2 for lung cancer immunotherapy.