Eosinophilic inflammation and bronchial mucus secretion are among the characteristic pathological changes in asthmatic reaction, which is mediated by Th2 type responses. Although it belongs to Th2 cytokines especially in the mouse, IL-10 is often considered an inhibitory cytokine for both Th1 and Th2 cells. In the present study, using a murine asthma model induced by ovalbumin (OVA), we demonstrated that endogenous IL-10 is critical for the development of asthma-like responses. Specifically, in comparison with wild-type controls, IL-10 gene knockout (KO) mice showed significantly reduced IL-5 production, eosinophilic inflammation and mucus production without notable changes in IL-4 and IgE responses following i. p. sensitization and subsequent intranasal challenge with OVA. In addition, Th1-related cytokine (IFN-gamma and IL-12) production in IL-10 KO mice was significantly higher than that in wild-type mice. The results suggest that endogenous IL-10 plays an important role in promoting pulmonary eosinophilic inflammatory reaction and mucus production during asthmatic reaction. The data also argue that IL-10 may be more influential in the development of IL-5-producing Th2 cells which differ from typical Th2 cells producing both IL-4 and IL-5.