A soluble form of the human IL-5R alpha-chain (IL-5Ra) that contains the extracellular IL-5 binding domain has been evaluated for its effect on IL-5 binding to and activation of human eosinophils and basophils. The truncated receptor was expressed in Escherichia coli and recovered in biologically active form following renaturation and anion exchange chromatography. The soluble receptor formed a 1/1 complex with IL-5 in solution and bound IL-5 with affinity comparable to that of cell-associated IL-5Ra. Soluble IL-5Ra also competed with IL-5 for binding to the native alpha beta IL-5R on human cells and inhibited IL-5-mediated receptor activation and inflammatory mediator production. In this regard, the soluble receptor prevented IL-5-induced tyrosine phosphorylation of JAK2 kinase and IL-5R beta-chain and inhibited IL-5 priming of leukotriene C4 release by human basophils. However, the E. coli-derived receptor failed to inhibit IL-5 in longer term assays, including eosinophil survival and TF-1 cell proliferation, possibly due to its propensity to aggregate in a time- and temperature-dependent manner. In contrast, we observed that a soluble IL-5Ra derived from baculovirus-infected cells was less prone to aggregate and effectively antagonized IL-5-induced cell proliferation and survival. These findings indicate that the extracellular portion of the human IL-5Ra chain can prevent association of IL-5 with cell surface receptors and can attenuate signal transduction, mediator release, and survival of inflammatory cells. As such, soluble IL-5R may be useful in treating diseases such as human asthma, in which pulmonary injury is associated with the activity of IL-5R-bearing cells.