Phytohemagglutinin (PHA)-derived T lymphoblasts or T cell clones from patients genetically deficient in IL-12R g 1 (IL-12R g 1 -/-) or IFN-+ R1 (IFN-+ R1 -/-) produced two-to threefold reduced IFN-+ levels compared to the corresponding cells from healthy individuals after anti-CD3 and PMA stimulation. Moderate IFN-+ production was observed in PHA-derived T lymphoblasts or T cell clones derived from healthy subjects in the presence of anti-IFN-+ R1 or anti-IL-12 mAb, whereas it was negligible in the presence of both mAb. However, when anti-IFN-+ R1 and/or anti-IL-12 mAb were added during restimulation, the cells produced normal levels of IFN-+ , indicating that both IFN-+ and IL-12 had an effect on the priming phase. Moderate production of IFN-+ was partially enhanced only in IFN-+ R1 -/-T cell clones generated in the presence of IL-12, but was almost completely abolished when IL-12R g 1 -/-and IFN-+ R1 -/-T cell clones were generated in the presence of anti-IFN-+ R1 or anti-IL-12 mAb, respectively. IL-4 production was enhanced in T cell clones from IL-12R g 1 -/-, but not from IFN-+ R1 -/-patients, whereas IL-10 and IL-2 production did not differ significantly in polyclonal T cells or clones from healthy and deficient individuals. These results indicate that IL-12R g 1-and IFN-+ R1-dependent signals co-ordinately regulate IFN-+ , but not IL-2 and IL-10 production, whereas only IL-12 negatively controls IL-4 production by in vitro-generated T cell clones. Thus, although IL-12 and IFN-+ signals are each sufficient for moderate production of IFN-+ by human T cells, both are needed for optimal IFN-+ production, and in the absence of both IFN-+ production is completely abrogated.