FcεRI, the high-affinity receptor for IgE, and eosinophils are thought to be key components of the allergic reaction underlying asthma and rhinitis. We provide evidence at the protein level that FcεRI is expressed in human blood eosinophils, and that the synthesis of FcεRI in purified human blood eosinophils is regulated by fibronectin in combination with IgE, IL-4, both involved in allergic reactions, and by RANTES, a strong chemotactic agent for eosinophils. This provides further evidence for a regulatory effect of IgE on human eosinophils in allergic disease.