A cDNA for a human interleukin3 (hlL-3) binding protein has been isolated by a novel expression cloning strategy: a cDNA library was coexpressed with the cDNA for the 6 subunit of human granulocytelmacrophage colony-stimulating factor (GM-CSF) receptor (hGMRf3) in COS7 cells and screened by binding of ‘251-labeled IL-3. The cloned cDNA (DUK-1) encodes a mature protein of 70 kd, which belongs to the cytokine receptor family and which alone binds hlL-3 with extremely low affinity (Kd= 120-+ 60 r&l). A high affinity IL-&binding site (&= 140 f 30 PM) was reconstituted by coexpressing the DUK-1 protein and hGMR3, indicating that hlL-3Fl and hGMR share the p subunit. Therefore, we designated DUK-1 as the a subunit of the hlL-3R. As in human hematopoietic cells, hlL-3 and hGM-CSF compete for binding in fibroblasts expressing the cDNAs for hlL-BRa, GMRa, and the common p subunit, indicating that different a subunits compete for a common p subunit.