Interleukin-12 (IL-12) is a cytokine that has regulatory effects on T and natural killer (NK) cells and is composed of two disulfide-bonded subunits, p40 and p35. It is a heterodimeric cytokine produced by macrophages, mitogen stimulated- or EBV infected-B lymphocytes, keratinocytes, and probably dendritic cells, with important immunoregulatory functions in vitro and in vivo. In its role as the initiator of cell-mediated immunity, it has been suggested that IL-12 has therapeutic potential as a stimulator of cell-mediated immune responses to microbial pathogens, metastatic cancers, and viral infections such as AIDS. Importantly, IL-12 also inhibited secondary granuloma formation in mice presensitized with eggs demonstrating a role for the cytokine in reversing established Th2-type responses.