The cellular events underlying the development and maintenance of allergic airway disease are complex. A number of different inflammatory cell types, including lymphocytes, mast cells, and eosinophils, are thought to play key roles in allergic airway disease. The accumulation of eosinophils in the airway is one of the hallmarks of asthma, and a large body of work has focused on understanding the biology of eosinophils in patients with asthma. Interleukin (IL)-5 is a crucial factor in the biology of eosinophils and has been shown to regulate a number of eosinophil functions, including apoptosis, chemotaxis, and exocytosis. In order to identify genes and proteins that are regulated by IL-5, we have performed proteomic and genomic analyses of human eosinophils that had and had not been treated with IL-5. In one set of experiments, eosinophils were prepared from the peripheral blood of five healthy donors and were stimulated in vitro with IL-5 or vehicle. Protein extracts were resolved on two-dimensional gels, and differentially expressed proteins were excised, trypsinized, and identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. In parallel experiments, microarrays (U95A; Affymetrix; Santa Clara, CA) were run on pooled RNA from primary human eosinophils that had and had not been treated with IL-5. Genes expressed in nonstimulated eosinophils were identified, as were subsets of genes that had been either up-regulated or down-regulated by IL-5 treatment. The results from the genomics analysis were compared with the results from the proteomics analysis.