Interleukin (IL)-12 was originally identified as a factor produced by human Epstein-Barr virus-transformed B cell lines. It was detected by one group as cytotoxic lymphocyte maturation factor, a cytokine that synergized with IL-2 in the induction of lymphokine-activated killer cells and cytotoxic T lymphocytes. A second group characterized it as a natural killer (NK) cell stimulatory factor, due to the enhancement of cytotoxicity and IFN-gamma synthesis by NK cells. Human IL-12 was purified to homogeneity and cloned by both groups. We had identified a murine factor, provisionally termed T cell-stimulating factor (TSF), which was involved in the proliferation, synthesis of IFN-gamma and cell adhesion of CD4+ Th1 cells. TSF was produced in the antigen-specific interaction between Th1 cells and macrophages as antigen-presenting cells, partially purified from supernatants of such cultures, and shown to be identical to IL-12. Monocytes/macrophages appear to be the major source of IL-12. It is rapidly produced by phagocytic cells after stimulation with several bacteria/bacterial products and other microorganisms. In the light of its effects on NK cells as well as CD4+ and CD8+ T cells, IL-12 can be regarded as a cytokine that connects the innate immune system with the acquired immunity. IL-12 has a broad range of activities already reviewed in three papers. These include the regulation of cytokine synthesis and proliferation of T and NK cells, the promotion of Th1 cell development, the differentiation of CD8+ T cells and effects on hematopoiesis. When applied in vivo, IL-12 was shown to enhance the resistance to bacterial and parasitic infections, to promote antitumor immunity, and to influence antiviral responses including HIV in vivo or in vitro. This review will briefly summarize these effects, but mainly focus on recent results concerning the regulation of the production and the activity of IL-12, its role in the differentiation of Th cells and the implications for delayed- and immediate-type hypersensitivity reactions, its importance for organ-specific autoimmune diseases, and the possible role of the IL-12p40 homodimer as a specific inhibitor of the IL-12 heterodimer.