Background Asthma is a chronic airway disease with inflammation characterized by physiological changes （airway hyper-responsiveness, AHR） and pathological changes （inflammatory cells infiltration and mucus production）. Eosinophils play a key role in the allergic inflammation. But the causative relationship between eosinophils and airway inflammation is hard to prove. One of the reasons is lack of activation marker of murine eosinophils. We investigated the expression of CD69 on murine eosinophils in vitro, the relationship between the expression of CD69 on eosinophils from peripheral blood and bronchoalveolar lavage fluid and on airway inflammation in asthmatic mice. Methods Eosinophils from peripheral blood of IL-5 transgenic mice （NJ.1638） were purified. Mice were divided into five groups： wild type mice sensitized and challenged with saline （WS group）, wild type mice sensitized and challenged with ovalbumin （WO group）, IL-5^-/- mice sensitized and challenged with saline and transferred with purified eosinophils （ISE group）, IL-5^-/- mice sensitized and challenged with OVA and transferred with purified eosinophils （IOE group）, IL-5^-/- mice sensitized and challenged with OVA and transferred with purified eosinophils, pretreated with anti CD4 monoclonal antibody （IOE＋antiCD4mAb group）. IL-5^-/- mice were sensitized with OVA at day 0 and day 14, then challenged with OVA aerosol. On days 24, 25, 26 and 27 purified eosinophils were transferred intratracheally to IL-5^-/- mice. On day 28, blood and BALF were collected and CD69 expression on eosinophils measured by flowcytometry. Results Purified eosinophils did not express CD69. But eosinophils cultured with PMA＋MA, IFN- T, IL-5 or GM-CSF expressed CD69 strongly. Eosinophils from blood of WO, WS group did not express CD69 at all. The numbers of eosinophils in BALF of WO group, IOE group, ISE group and IOE＋antiCD4mAb group were significantly higher than in mice of WS group which did not have eosinophils at all. CD69 expression
