TNF is effective in causing the regression of selected murine tumors when administered at high concentrations. Therapeutic levels in humans cannot be obtained systemically, however, because of dose-limiting toxicity. The development of immunotherapy with IL-2 and tumor-infiltrating lymphocytes (TIL), which can accumulate at tumor sites in some patients, and of efficient retroviral techniques for gene transfer into eukaryotic cells has allowed new therapeutic approaches using TNF. We have retrovirally transduced human TIL with the gene for TNF in an attempt to deliver high concentrations of TNF to the tumor site without dose-limiting systemic toxicity. Successful gene insertion was confirmed by Southern hybridization in 16 of 16 transduced and selected TIL cultures from 15 different patients, with an estimated 28 to 93% transduced cells within each culture. Transduced selected TIL cultures produced greater amounts of TNF, compared with nontransduced controls, in 11 of 16 cultures evaluated. However, overall production of TNF was > 30-fold lower, compared with a transduced and highly selected tumor cell line control (MEL-TNF). In addition, steady state levels of vector-derived transcript in nine of 10 transduced selected TIL cultures were < 14% of the amount seen in the MEL-TNF control line. In an attempt to increase TNF production, TIL were transduced with a mutated form of TNF containing the IFN-gamma signal peptide in place of the transmembranous region, to enhance secretion into the endoplasmic reticulum. By using this vector, TNF production increased by an average of fivefold. These studies demonstrate that TIL can be genetically modified to express and secrete a protein for use in targeted cancer therapy but that partial expression blockades exist that prevent maximal cytokine production by introduced genes in TIL.