Mast cells are exposed to an oxidative environment in the course of allergic and inflammatory reactions. We have examined the effects of H(2)O(2) stimulation in a primary rat basophilic leukemia cell line (RBL-2H3) and compared with IgE-dependent stimulation. Like IgE stimulation, H(2)O(2) up-regulates IL-4 and IL-6 gene expression and cytokine secretion, shows a little effect on IL-5 but does not induce IL-10 gene expression. Simultaneous H(2)O(2) treatment and FcepsilonRI triggering of mast cells has additive effects on IL-4 expression. In addition, we show that both stimuli induce the nuclear translocation of APE/Ref-1, a bifunctional enzyme that stimulates the DNA-binding activity of several transcription factors through the reduction of highly reactive cysteines. Conditional inactivation of APE/Ref-1 expression abolishes H(2)O(2)-induced IL-4 and IL-6 gene expression but does not affect that induced by FcepsilonRI stimulation. Our findings indicate that oxidative stress activates the gene expression of a specific cytokine pattern in mast cells through an APE/Ref-1-dependent pathway, which is distinct from the one that is activated by FcepsilonRI stimulation. Nonetheless, H(2)O(2) and FcepsilonRI signalings are additive in augmenting IL-4 production. Most importantly, oxidative stress can induce a pro-type 2 inflammatory response from mast cells that is independent of FcepsilonRI stimulation.