Interleukin-5 (IL-5) promotes the growth and differentiation of human eosinophils and may regulate the selective eosinophilia and eosinophil activation seen in certain diseases. Radiolabeled recombinant human IL- 5 (hIL-5) was used to characterize the IL-5 receptor present on normal human eosinophils and on the myeloid leukemia line HL-60, which can be induced to differentiate into eosinophilic cells. Binding studies with eosinophils and HL-60 cells grown under alkaline conditions demonstrated similar high-affinity binding sites for hIL-5 on both cell types with kd values of approximately 400 pmol/L. The binding observed was specific in that it was not inhibited by hIL-3, human granulocyte- macrophage colony-stimulating factor, or hIL-2. Binding studies with a number of other human cell lines, including a B-lymphoma line, and with lymphocyte and neutrophil preparations were also performed, but IL-5 receptors were not detectable on these cells. The number of hIL-5 receptors on HL-60 cells could be correlated with its propensity to differentiate towards an eosinophilic cell type. Expression of hIL-5 receptors on HL-60 cells was upregulated by butyric acid under alkaline conditions, downregulated by hIL-3, virtually eliminated by dimethyl sulfoxide and hIL-5, while hIL-2 had no detectable effect. One major 125I-hIL-5-crosslinked complex of 75 to 85 Kd in Mr was detected on HL- 60 cells using crosslinking agents giving a molecular mass of 55 to 60 Kd for the hIL-5 receptor itself. Studies using cellular autoradiography showed that IL-5 receptors were evenly distributed on eosinophils but that receptor distribution on HL-60 cells was noticeably heterogeneous. Eosinophils were the only cells in slides prepared from peripheral blood that had detectable levels of IL-5 receptors in agreement with the specific action of IL-5 on the human eosinophil lineage.