IL-12 is a potent regulator of cell mediated immune responses and it induces IFN-γ production by NK and T cells. It is produced by activated monocytes/macrophage cells, B lymphocytes and connective tissue type mast cells. Among its biological activities IL-12 promotes the growth and activity of activated NK, CD4+ and CD8+ cells and induces the development of IFN-γ producing Th1 cells. Recombinant murine IL-12 is a 75.0 kD heterodimeric glycoprotein consisting of disulfide-linked 35 kD (p35) and 40 kD (p40) subunits (506 total amino acid residues).