We have cloned the IL-2 receptor gene from human genomic DNA libraries using IL-2 receptor cDNA as probe. The genomic DNA segments that hybridized with cDNA were subcloned in M13 phages and their sequences were determined. The nucleotide sequences showed that the IL-2 receptor gene was encoded by eight exons and that the coding region sequences agreed completely with that of the IL-2 receptor cDNA cloned from a cell line derived from adult T cell leukemia (ATL), in which IL-2 receptors are expressed abnormally. The nucleotide sequence of the 5'-flanking region had a putative promotor region, which had some homology with the human IL-2 gene. Transcription initiation sites were clustered about 25 bp 3' to the TATA box as assessed by primer extension analysis. These sites for normal and ATL T cells were the same. Exons 2 and 4 encoding the extracytoplasmic portion had significant homology, suggesting that the two exons are derived by duplication of an ancestral exon. Exon 2 contained six cysteine residues, four of which are conserved at the corresponding positions in exon 4.