IL-12 is a cytokine that can exert regulatory effects on T and NK cells and promote Th1 responses. To delineate further the physiologic role of IL-12 in immunity, mice deficient for this cytokine were generated. IL-12-deficient mice were impaired but not completely lacking in the ability to produce IFN gamma following endotoxin administration and to mount a Th1 response in vivo, as measured by antigen-induced IFN gamma secretion by immune lymph node cells in vitro. In contrast, secretion of IL-4 was enhanced, while proliferation and secretion of IL-2 and IL-10 were normal following antigen stimulation. DTH responses were significantly reduced in IL-12-deficient mice, but no defect in allogeneic CTL responses was observed. These results indicate that IL-12 plays an essential role in regulating IFN gamma production and in facilitating normal DTH responses. However, other phenomena associated with Th1 responses and cell-mediated immunity, i.e., IL-2 secretion and CTL generation, were not compromised in the absence of IL-12.