Tumor-infiltrating lymphocytes (TIL) are cytotoxic T cells isolated from solid tumors and expanded in vitro in recombinant interleukin-2 (rIL-2). TIL have antitumor effects in murine models and in some patients with melanoma. In an effort to generate murine TIL with enhanced in vivo therapeutic efficacy, viable tumor cells were coinjected with a collagen matrix plus recombinant human IL-6 (rIL-6) subcutaneously into syngeneic mice to achieve sustained local concentrations of rIL-6 at the tumor site from which TIL were derived. In five separate experiments, single cell suspensions of tumors were admixed with either (a) Hanks' balanced salt solution (HBSS), (b) 2% (20 mg/ml) collagen matrix only, (c) 250 µg rIL-6 only, or (d) 250 µg rIL-6 in a 2% collagen matrix (prolonged release) before subcutaneous inoculation. These tumors were subsequently resected and TIL were isolated and expanded in vitro. TIL generated from tumors admixed with matrix plus rIL-6 were significantly more effective than TIL expanded from tumors admixed with HBSS (four of five experiments), TIL from tumors admixed with matrix only (five of five experiments), and TIL from tumors admixed with rIL-6 only (three of four experiments) in an established tumor treatment model. In no experiment was any other TIL culture superior to TIL grown from tumors augmented with collagen matrix plus rIL-6. These results suggest that strategies designed to increase the local concentrations of cytokines at tumor sites may lead to the generation of more potent TIL for clinical administration.