Interleukin 12 (IL-12) is a heterodimeric cytokine that is produced primarily by antigen-presenting cells and plays a primary role in the induction of cell-mediated immunity. This function is promoted by the IL-12 induced production of interferon-gamma (IFN-gamma) from both resting and activated NK and T cells, by the proliferative activity of IL-12 on activated NK and T cells, by enhancing the cytotoxic activity of NK cells, and by supporting cytotoxic T lymphocyte generation. IL-12 and IL-12-induced IFN-gamma promote the development of naive T cells into Th1 cells and the proliferation and IFN-gamma secretion by differentiated Th1 cells in response to antigen. IL-12 has been found to exhibit many of these activities in vivo, as well as in vitro, and thus IL-12 plays an important role in both innate resistance and antigen-specific adaptive immunity to intracellular bacterial, fungal, and protozoan pathogens. Due to its effects on T cells, recombinant IL-12 has been shown to have therapeutic activity in a variety of mouse tumor and infectious disease models and is being evaluated in clinical trials in human cancer patients. IL-12 also appears to play a role in the genesis of some forms of immunopathology, including endotoxin-induced shock and some autoimmune diseases associated with aberrant Th1 activity. Therefore, IL-12 antagonists may also have therapeutic potential in the treatment of auto immune disorders.