Interleukin 12 (IL-12) is a heterodimeric pleiotropic 70 kD (p70) cytokine consisting of a 40 kD (p40) subunit and a 35 kD (p35) subunit. IL-12 is a potent regulator of cell-mediated immune responses and key mediator of Th1 cell development. Bioactive IL-12 is secreted by activated B lymphocytes, dendritic cells, and macrophages in response to pathogens or upon interaction with activated T cells.IL-12 p70 stimulates cytotoxic T, NK and LAK cell proliferation, andinduces IFN-gamma and TNF production in resting and activated T and NK cells.It has been suggested that IL-12 may have therapeutic potential as a vaccine adjuvant that promotes cellular-immunity and as an anti-tumor and anti-viral agent.