Vaccinia virus is the prototype orthopoxvirus that has been used as a vaccine strain for small pox. This virus has been used to express a variety of cellular and viral genes in mammalian cells at high levels. Interleukin-4 (IL-4) has been found to stimulate the proliferation of T cells and enhance the cytolytic activity of cytotoxic T lymphocytes. To test the immunotherapeutic potential of IL-4 delivered in vivo by poxvirus, a recombinant vaccinia virus expressing the murine IL-4 gene (RVVmIL-4) was constructed. A high level of IL-4 production was confirmed by infecting HeLa cells and measuring IL-4 in cell culture supernatant by ELISA. As a tumor model, two cell lines were used; the murine T leukemic line P388 and the murine breast cancer line TS/A. CDF1 mice were intraperitoneally inoculated with $1;{ imes};10^5$ cells of P388. Mice were injected at the same site with $5;{ imes};10^5;PFU$ of recombinant vaccinia virus; first, 3 days after the injection of tumor cells and thereafter once every week for 3 weeks. Intraperitoneal injections of RVVmIL-4 significantly prolonged the survival time of mice inoculated with tumor cells. All mice injected with RVVmIL-4 remained alive for 30 days after the postinoculation of tumor cells, while 100% and 70% of the animals injected with saline or wild type vaccinia virus died, respectively. In another tumor model using TS/A, tumor was established by subcutaneously inoculating $2{ imes}10^5$ tumor cells to BALB/c mice. After tumor formation was confirmed on day 4 in all mice, $5;{ imes};10^6;PFU$ of RVVmIL-4 was inoculated subcutaneously three times, once every week for 3 weeks. The TS/A tumor was eradicated in two of the nine mice. Seven of the nine mice treated with RVVmIL-4 developed a tumor, but tumor growth was significantly delayed compared to those treated with saline or wild type vaccinia virus. These results indicate that recombinant vaccinia viruses may be used as a convenient tool for delivering immunomodulator genes to a variety of tumors.