Interleukin-12 (IL-12) is a critical cytokine representing the link between the cellular and humoral branches of host immune defense apparatus. IL-12-induced cytotoxic lymphocyte (CTL) development is a central mechanism in immune responses against intracellular infectious agents as well as malignant growth. However,the molecular basis of tumor-specific CTL responses mediated by IL-12 remains poorly defined. In this study,we addressed this issue in a comprehensive manner to probe into IL-12-induced anti-tumor responses by global gene expression profiling of mRNA expression in CD8+T cells in a transplantable syngeneic mouse mammary carcinoma model treated or not with recombinant IL-12. A strong tumor regression was induced by the IL-12 treatment. An introspection of differential gene expression at an early stage of the IL-12-initiated CTL activation reveals interesting genes and molecular pathways that may account for the marked tumor regression,and is likely to provide a rich source of potential targets for further research and development of effective therapeutic modalities. Cellular & Molecular Immunology. 2004;1(5):357-366.