A major obstacle limiting the efficacy of adoptive T-cell transfer (adoptive immunotherapy) to treat patients with cancer is the short survival of the transferred cells. These in vitro activated T cells depend on the growth factor, interleukin (IL)-2, and may undergo apoptosis in vivo when they are transferred. The authors previously reported that the need for an exogenous source of IL-2 could be abrogated in vitro by retrovirally transducing antitumor T lymphocytes with an exogenous IL-2 gene. Here they report that this growth of IL-2 transductants depended on restimulation of the T-cell receptor complex and appeared to be regulated at the transcriptional level of the transduced IL-2 gene. The transduced IL-2 transcript was barely detectable in IL-2-transductants just before they died without restimulation, and they expressed a low level of the CD25 molecule, the alpha chain of the IL-2 trimeric receptor complex. Melanoma-specific tumor-infiltrating lymphocytes (either bulk or CD8+ cells alone), when transduced with an IL-2 retroviral vector, could produce IL-2 upon tumor stimulation and proliferated after the destruction of autologous tumor cells in the absence of added IL-2. Control vector-transduced tumor-infiltrating lymphocytes failed to do so under the same conditions. These findings provide a foundation for the development of clinical efforts to adoptively transfer melanoma-specific tumor-infiltrating lymphocytes transduced with an IL-2 retroviral vector for the treatment of patients with metastatic melanoma to evaluate the fate and therapeutic effect of these IL-2 gene-modified antitumor T lymphocytes in vivo.