B ecause T cell-derived cytokines may affect the functioning of eosinophils, we have investigated the capacity of human eosinophils to respond to IL-2. IL-2 was a potent chemoattractant with ED50 of 10(-12) M with eosinophils from all normal and eosinophilic donors tested. The monoclonal antibodies anti-Tac and TU27 against p55 (Tac/CD25) and p75 receptor subunits, respectively, each inhibited IL-2-dependent eosinophil migration. The molar potency of IL-2 and the inhibitory activity of each of the above antibodies suggest that high affinity heterodimeric IL-2 receptor complexes mediated the migration responses of eosinophils to IL-2. Binding of monoclonal antibody against p75 was not detectable by flow cytometry, and high affinity binding sites for 125I-IL-2 were below the limits of quantitation on eosinophils from individuals with eosinophilia. Expression of p55 (Tac/CD25) by eosinophils, without requirement for in vitro activation, was demonstrable by flow cytometry, radioimmunoprecipitation, and Northern blotting for mRNA. Surface expression of p55 on eosinophils from normal or eosinophilic individuals increased during culture for 24-48 h with a biologic activity purified from stimulated U937 cells and to a lesser extent with granulocyte-macrophage CSF or lymphocyte chemoattractant factor but not with nine other cytokines. These studies indicate that blood eosinophils respond to IL-2 and identify one mechanism whereby activation of T lymphocytes may influence the function of eosinophils.