Using non-stringent hybridization with a human interleukin-2 cDNA probe, we have isolated recombinant phages from a mouse genomic DNA library cloned in the EMBL3 phage. The sequence and organization of the mouse interleukin-2 (IL-2) gene was determined. By comparison with the human IL-2 sequence, three introns can be identified with lengths of 99, +/- 2 400, and +/- 1 900 base pairs, respectively. The mouse IL-2 gene codes for a polypeptide of 169 amino acids and contains a putative signal peptide of 20 amino acids. The homology to the human interleukin-2 is 72% at the nucleotide level in the coding part and 65% at the amino acid level. An extraordinary sequence, consisting of 12 consecutive CAG codons coding for glutamine, is found in the first exon.