We have amplified by PCR and sequenced the first exon of the interleukin 2 gene from the RF/J mouse strain DNA. When we compared the RF/J first exon sequence with the one reported previously, we found several differences. These differences are also reflected in the deduced amino acid sequence and they have been localized in the first 23 amino acids of the mature polypeptide. The finding of this new IL-2 sequence shows that there is more than one allele for the mouse IL-2 molecule and raises the possibility of functional differences between alleles.