Transduction of the human interleukin-2 (IL-2) gene into tumor cells was carried out in order to develop a new immunotherapy for advanced head and neck carcinomas with a poor outcome. We transduced the IL-2 gene into KB cells, a head and neck squamous cell carcinoma cell line, using a defective herpes simplex viral (HSV) amplicon vector as a gene transfer vehicle. A high level of IL-2 was secreted by IL-2 gene-transduced KB cells (KB/IL-2). The IL-2 producibility of irradiated KB/IL-2 cells was almost the same as that of non-irradiated cells. In the tumor establishment model in nude mice, IL-2 and interferon-gamma (IFN-gamma) at high concentrations were detected in the sera of mice transplanted with KB/IL-2 cells. The spleen cells of nude mice transplanted with KB/IL-2 cells exhibited high cytotoxic activity compared to those from mice transplanted with KB cells and from untreated mice. Three of five mice transplanted with KB/IL-2 cells rejected tumors. In the treatment of established tumors, therapeutic effects due to irradiated KB/IL-2 were dose-dependent. The suppressive effects on tumor growth were blocked by anti-asialo GM1, anti-human IL-2 and anti-IFN-gamma antibodies. Immunohistochemical observation revealed the presence of asialo GM1(+) cells among the KB/IL-2 cells in tumors transplanted into nude mice.