Cytokine gene transfer to tumor cells can augment host antitumor responses and modify tumor phenotype. To evaluate the immunoregulatory and antitumor capacities of lung tumor-derived interleukin-7 (IL-7), we transduced non-small-cell lung cancer (NSCLC) cell lines with the IL-7/HyTK internal ribosomal entry site (IRES) retroviral vector and evaluated modifications in tumor phenotype and cocultured effector activities. In vitro proliferation of IL-7-transduced tumor cells was significantly less than control vector-transduced and parental tumor cells. The decreased proliferation rates of IL-7-transduced cells could be reproduced by adding high concentrations of recombinant IL-7 to the parental cells. Anti-IL-7 monoclonal antibody significantly increased the proliferation of the IL-7-transduced cells (P < .05). Parental NSCLC cells were found to express the IL-7 receptor, and IL-7 gene transduction did not alter expression of the IL-7 receptor. IL-7 transduction significantly altered tumor cell expression of intracellular adhesion molecule 1, major histocompatibility complex 1, lymphocyte function-related antigen 3, very late activation antigen beta 1, and p185neu. Peripheral blood lymphocytes cocultured with either IL-7-transduced tumor cells or tumor supernatants had enhanced cytolytic and proliferative capacities compared with coculture with control vector-transduced or parental cells. Our findings indicate that IL-7 gene transfer in NSCLC significantly augments cocultured effector activities in vitro, inhibits tumor cell proliferation, and modifies tumor cell surface phenotype. These findings suggest that IL-7 gene therapy may be effective in modifying host antitumor responses in NSCLC.