We have been studying mechanisms of how recall responses by memory B (Bmem) cells are regulated. RT-PCR analyses demonstrated that IL-9R mRNA is highly expressed Bmem cells in mice during T cell-dependent (TD) immune response. In this study, we analyzed IL-9Rα deficient (Il9r–/–) mice to examine the role of the IL-9 pathway in the TD immune response. Using our original monoclonal antibody, we analyzed surface expression of IL-9R on ex vivo spleen cells of immunized mice, in comparison with Il9r–/– mice, by flow cytometry. IL-9R was clearly detectable on Bmem cells at 3 weeks after immunization onward, but not on other cells except mast cells. We found that Il9r–/– mice normally produced antigen-specific IgG1 antibody after the primary immunization, but exhibited significantly less IgG1 antibody production and attenuated expansion of antigen-specific B cells after the secondary immunization. The attenuated recall response was caused by B-cell intrinsic defect. Moreover, we demonstrated that stimulation through IL-9R facilitated Bmem cell proliferation and differentiation to plasma cells in vitro. By contrast, the number of antigen-specific IgG1+ germinal-center (GC) B cells generated after secondary immunization was significantly higher in Il9r–/– mice than in Il9r+/+ mice. These results indicate that the signal through IL-9R on Bmem cells facilitates their proliferation and differentiation to antibody-producing cells, whereas attenuates their differentiation to GC B cells, during the recall response. This study has revealed a critical cytokine that regulates expansion and differentiation of memory B cells upon re-exposure to antigen.