A number of lung diseases, including many interstitial lung diseases and HIV infection, are associated with decreases in intracellular thiols. Altered Th1/Th2 T cell balance has also been associated with disease progression in many of the same diseases. IFN-gamma and IL-4 are critical effector cytokines of Th1 and Th2 cells, respectively. To determine the effect of thiols on the production of IFN-gamma and IL-4 by splenocytes, cells were incubated in the presence and the absence of N-acetylcysteine (NAC) and stimulated with alphaCD3 or alphaCD3 and IL-12. Augmenting intracellular soluble thiol pools ( approximately 2-fold) with 15 mM NAC blocked induction of IFN-gamma and increased production of IL-4 without causing significant changes in intracellular glutathione levels. The effect of NAC on IL-4 production was not linked to an increase in STAT6 phosphorylation, as STAT6 levels were decreased, nor did the increase in IL-4 occur with purified CD4 cells. We found that NAC increased splenocyte IL-4 production via an effect on APCs. We also found that NAC increased two IL-4 relevant transcription factors (AP-1) and NFATc. These studies suggest that increasing intracellular reduced thiol pools decreases IL-12 signaling and IFN-gamma production, while increasing IL-4 production. The sum of these effects may contribute to alterations in the balance between Th1 and Th2 responses in lung diseases associated alterations in intracellular thiol pools.