Biologically active IL-12 is a 70kDa heterodimer (p70) consisting of two covalently linked subunits, p35 and p40. The main producers of this cytokine are antigen-presenting cells, specifically dendritic cells and macrophages. IL-12 effects are primarily controlled at the level of p40 transcription and IL-12R expression (only the $2 subunit has signal-transducing capacity). IL-12 is rapidly produced after infection and acts as a proinflammatory cytokine by inducing IFN7 production and enhancing proliferation and cytotoxicity of NK and T cells. IL-12 promotes TH1 immune responses, including the induction of TH1-mediated autoimmune diseases. IL-12 also enhances resistance to a variety of infectious diseases, acts as an adjuvant in vaccination, and exhibits potent antitumor immunity. Clinical trials have resulted in some promising responses. The current necessity of proper dosing schedules and of evaluating IL-12 administration resulted in combination with other cytokines, including IL-2 and IL-18.