IL-12 is a heterodimeric cytokine that promotes cell-mediated immunity through its regulatory effects on T and NK cells. In some murine infectious disease models, IL-12 was shown to be produced endogenously in response to infection, and the exogenous administration of IL-12 to mice with either infectious diseases or tumors has resulted in significant therapeutic effects. IL-12 was protective early in the disease process, as well as against established disease. However, the biologic activities of IL-12 that are beneficial in the host response to these infectious diseases and malignancies can also be deleterious in certain disease states. Thus, IL-12 has considerable potential for the treatment of a variety of human disorders if used under the appropriate conditions. Likewise, antagonists of IL-12 may have a role in controlling diseases with pathologies that are mediated through immune mechanisms.