Antigen-induced eosinophil recruitment into the airways of sensitized mice is mediated by CD4(+) T cells and their cytokines, especially IL-5. In this study, we found that the antigen-induced airway eosinophilia was diminished in Stat5a-deficient (Stat5a(-/-)) mice and Stat5b-deficient (Stat5b(-/-)) mice. We also found that antigen-induced CD4(+) T-cell infiltration and IL-5 production in the airways were diminished in Stat5a(-/- )mice and Stat5b(-/-) mice. Moreover, antigen-induced proliferation of splenocytes was diminished in Stat5a(-/- )mice and Stat5b(-/-) mice, suggesting that the generation of antigen-primed T cells may be compromised in Stat5a(-/-) mice and Stat5b(-/-) mice and this defect may account for the diminished antigen-induced T-cell infiltration into the airways. Interestingly, IL-4 and IL-5 production from anti-CD3-stimulated splenocytes was diminished in Stat5a(-/-) mice and Stat5b(-/-) mice. However, antigen-specific IgE and IgG1 production was diminished in Stat5a(-/-) mice but not in Stat5b(-/-) mice, whereas antigen-specific IgG2a production was increased in Stat5a(-/-) mice, suggesting the enhanced Th1 responses in Stat5a(-/-) mice. Finally, we found that eosinophilopoiesis induced by the administration of recombinant IL-5 was also diminished in Stat5a(-/-) mice and Stat5b(-/-) mice. Together, these results indicate that both Stat5a and Stat5b are essential for induction of antigen-induced eosinophil recruitment into the airways and that the defects in antigen-induced eosinophil recruitment in Stat5a(-/-) mice and Stat5b(-/-) mice result from both impaired IL-5 production in the airways and diminished IL-5 responsiveness of eosinophils. (Blood. 2000;95:1370-1377)