Interleukin-3 (IL3) and granulocyte/macrophage colony-stimulating factor (CSF2) stimulate proliferation and differentiation of various hemopoietic cell types. As is characteristic of the cytokine receptor family, the receptors for these proteins comprise alpha- and beta-subunits. While IL3 and CSF2 receptors each have unique alpha- subunits, they share a common beta-subunit. By Southern analysis of somatic cell hybrid panels, pulsed-field gel electrophoresis (PFGE), and fluorescence chromosomal in situ hybridization, we have mapped the cloned sequence for the IL3 receptor alpha (IL3RA) to the X-Y pseudoautosomal region at bands Xp22.3 and Yp11.3, near the gene for the alpha-subunit of the CSF2 receptor (CSF2RA). The CSF2RA and IL3RA genes are so close that their order could not be determined by two-color interphase in situ hybridization. They share PFGE fragments generated by different restriction enzymes down to the 50-100-kb size range. Pseudoautosomal inheritance was demonstrated by an EcoRI RFLP detected with the IL3RA cDNA probe.