We have shown that a T-cell clone derived from murine tumorinfiltrating lymphocytes (TILs) can be established that mediates in vitro and in vivo antitumor effects. Utilizing this clone as a model, we examined the effect of cytokines on T-cell antitumor effector mechanisms in vitro and in vivo. This clone, termed BF-1, was generated by limiting dilution culture of a freshly excised MC-38 tumor, growing it in low levels of interleukin-2 (IL-2), and has been maintained for over 600 days. This clone became specifically cytotoxic for the MC-38 tumor during its first 100 days of culture. Pretreatment of the parental MC-38 tumor cell line with tumor necrosis factor (TNF) and interferon-[gamma](IFN-[gamma]) increased its susceptibility to lysis by the BF-1 TIL line, but not to lysis by lymphokine-activated killer cells, in in vitro cytotoxicity assays. This increased susceptibility of the cytokine-pretreated targets was restricted to the parental tumor (MC-38), since similar pretreatment of MCA-102, MCA-105, or MCA-106 tumors did not render them susceptible to lysis by BF-1 TILs. This increased sensitivity to lysis in vitro was not the result of a change in the expression of major histocompatibility complex class I molecules. In experiments testing the ability of TILs to treat established lung metastases, the combination of TNF, IFN-[gamma], IL-2, and TILs was shown to increase significantly the antitumor properties of this therapy when compared to TILs and IL-2. This result demonstrates that combinations of lymphokines, which when administered alone do not affect micrometastatic tumor burdens (TNF, IFN-[gamma]), can synergize with cellular immunotherapy in the treatment of established tumor burdens and may have applicabilities to the treatment of cancer in humans.