N-acetyl-L-cysteine (NAC) is a thiol-containing compound and acts as a precursor for glutathione (GSH). It behaves as an antioxidant in mammalian cells and also exerts anti-inflammatory effects. NAC is also known to affect several immune cells including eosinophils, B cells, T cells, and dendritic cells (DC) in many aspects. Even though it has been reported that NAC inhibits DC activation and shifts the immune response to Th2, these studies exhibit some contradictory results in detail and do not give any information with respect to the induction of regulatory T cells. In this study, we re-analyzed the effects of NAC on DC during their activation. We also evaluated whether it induced T cell anergy, Th1/Th2 shift, or regulatory T cells. NAC suppressed the elevation of intracellular reactive oxygen species during DC activation. In parallel, it down-regulated surface expression of CD40 and CD86, suppressed the decrease of phagocytic function, lowered the secretion of cytokines such as IL-6, IL-10, and IL-12. All these effects showed dose-dependency. Thus, it seems likely that NAC inhibited DC activation with regard to their phenotype and cytokine secretion. When we evaluated the T cell-stimulating capacity of these NAC-DC, T cell proliferation and secretion of both Th1 (IFN-gamma) and Th2 cytokine (IL-5) were decreased. This implies that the T cell-stimulating activity of NAC-DC decreased without any shift to Th1 or Th2 cytokine (IL-5). The secretion of IL-10 and TGF-beta in the supernatants were also decreased, which suggests that the decrease of T cell proliferation and cytokine secretion is due to the induction of T cell anergy, rather than regulatory T cells.