To determine whether the costimulatory signal via CD28/B7 interaction is required for causing allergic inflammation, we examined the effect of administration of CTLA4-Ig, a fusion protein of the extracellular domain of CTLA4 and human IgG1-constant region, at the time of sensitization, on antigen-induced eosinophil infiltration in the trachea of sensitized mice, on IL-2, IFN-gamma, IL-4 and IL-5 production in the airways of the mice and on antigen-specific IgE synthesis in the mice. Administration of CTLA4-Ig at the time of sensitization suppressed antigen-induced eosinophil infiltration into the trachea and antigen-specific IgE production in mice. Furthermore, CTLA4-Ig administration at the time of sensitization suppressed not only IL-2 production but also IFN&hyphengamma and Th2 cytokine IL-4 and IL-5 production in the airways. Because allergic inflammation requires CD4+ T cells producing Th2-type cytokines IL-4 and IL-5, our results suggest that the costimulatory signal via CD28/B7 interaction is important for the generation and activation of Th2 cells and thereby for the development of allergic inflammation.