Summary cDNA clones encoding two receptor proteins involved in the binding of human interleukin 5 (hlL5) have been isolated. A first class codes for an IL5specific chain (hILLflu). The major transcript of this receptor gene, as analyzed in both HL-60 eosinophilic cells and eosinophilic myelocytes grown from cord blood, encodes a secreted form of this receptor. This soluble hlL5Ra has antagonistic properties. A second component of the hlL5R is found lo be identical to the p chain of the human granulocyte-macrophage colony-stimulating factor (GM-CSF) high affinity receptor. The finding that IL5 and GM-CSF share a receptor subunit provides a molecular basis for the observation that these cytokines can partially interfere with each other’s binding and have highly overlapping biological activities on eosinophils.