IFN-gamma contributes to the rejection of transplantable tumors and the inhibition of methylcholanthrene (MCA)-induced carcinogenesis by different mechanisms. In most tumor transplantation models, tumor rejection requires IFN-gamma receptor expression by host cells, but not by tumor cells. IFN-gamma produced by either CD4+ or CD8+ T cells acts on non-hematopoietic tumor stroma cells and, either directly or indirectly, induces angiostasis. This prevents rapid tumor burden and allows residual tumor cells to be eliminated. In some models, IFN-gamma also contributes to the destruction of existing tumor blood vessels. During MCA-induced tumorigenesis IFN-gamma is involved in the inhibition of MCA diffusion by encapsulation and reduction of DNA damage. This mechanism may primarily protect tissue from damage and simultaneously inhibit tumor development.