Mouse interleukin 12 (IL-12) is also known as natural killer cell stimulatory factor (NKSF) or cytotoxic lymphocyte maturation factor (CLMF). The biologically active mouse IL-12 is a disulfide-linked, 70 kDa (p70) heterodimeric glycoprotein composed of a 40 kDa (p40) subunit and a 35 kDa (p35) subunit. While the p40 and p35 subunits by themselves do not have IL-12 activity, the p40 homodimer has been shown to bind the IL-12 receptor so that it is an IL-12 antagonist. IL-2 is a pleiotropic cytokine which has been shown to have multiple effects on T lymphocytes and natural killer (NK) cells. Some of these effects include the induction of IFN-γ and TNF production by T and NK cells, the enhancement of cytotoxic activity of T and NK cells and the stimulation of T and NK cell proliferation. IL-12 has also been shown to be a central mediator of the cell-mediated immune response by promoting Th1 development. Although mouse IL-12 is active on both human and mouse cells, human IL-12 is only active on human cells. IL-12 is produced by macrophages, dendritic cells, monocytes, Langerhans cells, neutrophils, and keratinocytes.