Tumor-infiltrating lymphocytes (TIL) are a heterogeneous population of T cells with potent antitumor activity against a wide variety of tumors. TIL from renal cell cancer (RCC) typically exhibit diminished growth and antitumor activity after four weeks in vitro. We have therefore investigated effects of varying doses of interleukin-6 (IL-6) (0, 25, 100 units/ml.) on in vitro expansion, proliferation, cytotoxicity, and expression of cell surface phenotypes of long term renal TIL cultures from three RCC patients. Among the various conditions tested, three of three TIL cultures displayed a mild increase in cell expansion when grown in IL-2 with the addition of 100 units/ml. of IL-6. Two of three TIL cultures grown in IL-2 and 100 U/ml. of IL-6 demonstrated enhanced proliferation as determined by 3H-thymidine uptake. TIL could not be isolated or maintained in vitro when grown in the presence of IL-6 alone without IL-2. IL-6 was also found to enhance the long term non-specific cytotoxicity against an allogeneic nonrenal tumor target. No consistent effect on autologous tumor-specific cytotoxicity was demonstrated. We conclude that IL-6, when used in combination with IL-2, may modestly enhance the long-term growth of RCC-derived TIL.