IL-12 is produced by macrophages and B?lymphocytes and has been shown to have multiple effects on T?cells and natural killer?(NK) cells. These effects include inducing production of IFN-gamma and TNF by resting and activated T?and NK?cells, synergizing with other IFN-gamma ?inducers at both the transcriptional and post-transcriptional levels.?This interaction induces IFN-gamma ?gene expression, enhancing the cytotoxic activity of resting NK and T?cells, inducing and synergizing with IL-2 in the generation of lymphokine-activated killer?(LAK) cells, acting as a co-mitogen to stimulate proliferation of resting T cells, and inducing proliferation of activated T?and NK?cells. Current evidence indicates that IL?12, produced by macrophages in response to infectious agents, is a central mediator of the cell?mediated immune response by its actions on the development, proliferation, and activities of TH1?cells. 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.