The human gene for interleukin 2 (IL2) was assigned to chromosome 4 using human-mouse somatic cell hybrids and Southern filter hybridization of cell hybrid DNA. To identify IL2, a recombinant DNA probe ( pIL2 - 50A ) was used which contained a human interleukin 2 cDNA insert which hybridized to a 3.5-kb fragment in human DNA when cleaved with the restriction enzyme EcoRI.