A recombinant vaccinia virus expressing murine granulocyte-macrophage colony-stimulating factor (VVGM-CSF) was tested for its antitumor activity. Murine pulmonary metastasis was established by injecting 2×10⁵ B16FI0 melanoma cells into the tail vein of C57BL/6 mice. Three days after BI6F10 inoculation, WGM-CSF or VVTK, a thymidine kinase gene deficient control vaccinia virus, were injected intraperitoneally twice weekly for 2 weeks. Two weeks later, the mice were sacrificed and pulmonary metastasis foci counted. The results demonstrated that VVGM-CSF treatment significantly decreased the number of pulmonary metastasis and prolonged the survival time of tumorbearing mice. Cytotoxic and phagocytic activities of the peritoneal macrophages were found to be markedly elevated in mice treated with WGM-CSF. Nitric oxide released from the macrophages was also found to be increased. These data, together with our other results, strongly demonstrated that continuous secretion of GMCSF and activation of macrophages might partially explain the therapeutic effects of WGM-CSF on murine pulmonary metastasis.