Sputum from symptomatic asthmatics is a rich source of eosinophils from the respiratory tract. Following liquefaction of sputum with dithioerythritol (DTE), a cell suspension suitable for indirect double immunofluorescence with flow cytometry was obtained. Eosinophils were identified using anti-CD9 fluorescein conjugate, and particular surface markers measured with the relevant mouse MoAb followed by goat anti-mouse immunoglobulin phycoerythrin conjugate. Blood and sputum eosinophil surface markers were determined in parallel from asthmatics not receiving steroid therapy. Sputum eosinophils were found to have considerably elevated levels of CDl1b, a reflection of eosinophil activation. Sputum but not blood eosinophils were found to express ICAM-I (nine out of 11 cases) and HLA-DR (eight out of 11 cases). Furthermore, following culture of normal blood eosinophils with pooled T cell supernatants, ICAM-l and HLA-DR could be induced in vitro. The induction of eosinophil adhesion molecules such as ICAM-l and HLA-DR may influence eosinophil localization and function in asthma.