Studies of bronchoalveolar lavage (BAL) fluid from patients with allergic asthma have demonstrated active migration of eosinophils into the bronchial lumen after allergen challenge. The mechanisms mediating this eosinophil infiltration and cell activation are largely unexplained. The expression of several cell-surface molecules was measured on eosinophils derived from blood and BAL fluid 4 h after an allergen-induced early asthmatic reaction in order to find indications for a role of these molecules during extravasation to and activation in the bronchial compartment. Nine patients with allergic asthma participated in the study. An eosinophil-specific, high-depolarization signal enabled us to measure expression on eosinophils in a fluorescence activated cell sorter (FACS) analysis without isolation of these cells. Eosinophils recovered from BAL showed a different phenotype than blood eosinophils; upregulation of CR-3, p150/95, CD67, and CD63, and downregulation of L-selectin indicate that the cells are activated in terms of degranulation. Up-regulation of intercellular adhesion molecule-1 (ICAM-1), LFA-3, and human leukocyte antigen II (HLA-II) might enable cell-cell contact between T-lymphocytes and eosinophils, probably leading to immunomodulation and cell activation. The finding that eosinophils in BAL are activated and can interact with T cells is further evidence for the proinflammatory role of these cells in allergic asthma.