The antigen, CD69, has been shown to be expressed on activated lymphocytes. CD69 has also been shown to be associated with signal transduction process. Recently, we demonstrated the expression of CD69 on eosinophils at inflammatory sites. Whatever the disease, lung eosinophils obtained from the bronchoalveolar lavage fluid or pleural effusion expressed significant levels of CD69, whereas most peripheral blood (PB) eosinophils did not express CD69. Further, in vitro stimulation of PB eosinophils by IL-3, IL-5, GM-CSF and PMA induced CD69 expression on PB eosinophils. These findings suggest that CD69 expression on eosinophils is restricted to local inflammatory sites and CD69 may be a useful marker for activated eosinophils at inflammatory sites.