To obtain information about useful combinations of various cytokines in melanoma therapy, we studied the influence of interleukin-2 (IL-2) in combination with interferon-alpha (IFN-alpha), IFN-gamma and tumour necrosis factor-alpha (TNF-alpha) on the lytic activity of IL-2-stimulated cells in vitro. Peripheral mononuclear cells (PMC) were incubated for 4 days with various combinations of cytokines and used as effector cells. Two different melanoma cell lines (M19 and M26) were used as target cells. The lytic activity of stimulated PMC was determined using a modified hexosaminidase assay. IL-2 was mainly responsible for the lytic activity of the effector cells in a dose-dependent manner. IFN-alpha, IFN-gamma and TNF-alpha did not enhance lytic activity with an optimal IL-2 dose (50 IU[ml IL-2). Using a suboptimal IL-2 dose (5 IU/ml), they increased cytotoxicity. The specific lysis of M19 cells was significantly increased by pretreatment of the cells with 5 IU/ml IFN-alpha together with 50 IU/ml TNF-alpha (t-test, P < 0.001), while the specific lysis of M26 cells was increased by pretreatment with 5 IU/ml IFN-gamma. We conclude that the lysis of melanoma cells by cytotoxic cells in vitro can be enhanced by various cytokines. The optimal cytokine combination differed for the two melanoma cell lines tested.