In this report it is shown by immunofluorescence analysis, biochemical analysis and mRNA hybridization that human eosinophils express surface CD4 and interleukin-2 receptor (IL-2R) (CD25) when exposed to eosinophil activators granulocyte-macrophage colony-stimulating factor (GM-CSF) and IL-3. Although the functional role of eosinophil CD4/CD25 expression has to be elucidated, it will be of interest in further studies to investigate whether in vivo induction of these molecules occurs in association with certain disease processes such as the hypereosinophilic syndrome or in immunological responses during allergic and helminthic parasitic diseases.