Interleukin-12 (IL-12) is a heterodimeric 70 kDa glycoprotein (IL-12 p70) consisting of a 40 kDa subunit and a 35 kDa subunit linked by disulfide bonds that are essential for the biological activity of IL-12. p40 has been shown to be a subunit of another composite cytokine, designated IL-23. IL-12 is produced by activated hematopoietic phagocytic cells (monocytes, macrophages and neutrophils) and by dendritic cells. IL-12 acts like a growth factor for activated T and NK cells and enhances the lytic activity of NK/lymphokine-activated killer cells. IL-12, in conjunction with the other IL-12 family members IL-23 and IL-27, promotes the development of a CD4+ Th1 immune response. IL-12 has been shown to inhibit the growth of a variety of experimental tumors and to have anti-angiogenic effects in vivo.