Granulocyte-macrophage colony-stimulating factor (GM-CSF)-transduced B16-F10 murine melanoma cells had lower tumorigenicity in both syngeneic and nude mice than parental or LacZ-transduced (control) cells. The subcutaneous (s.c.) tumors producing GM-CSF were densely infiltrated with macrophages, whereas the control tumors were not. In vitro studies showed that GM-CSF-transduced B16 cells were susceptible to lysis mediated by nonactivated murine macrophages, whereas control B16 cells were not. Macrophage-mediated cytotoxicity against GM-CSF-transduced B16 cells was independent of the presence of NO, H2O2, O2-, tumor necrosis factor alpha, and matrix metalloproteinase. Coculture experiments using GM-CSF-producing and -nonproducing B16 cells demonstrated that GM-CSF produced by the transduced B16 cells activated macrophages to kill the bystander non-GM-CSF-producing tumor cells. The results suggest that GM-CSF released by tumor cells can induce macrophage-mediated cytotoxicity, which in turn can inhibit the in vivo growth of GM-CSF-transduced tumor cells.