The gene encoding the 20-kDa glycoprotein of the T3-T-cell receptor complex (T3-delta chain) has been mapped to human chromosome 11 by hybridization of a T3-delta cDNA clone (pPGBC#9) to DNA from a panel of human-rodent somatic cell hybrids. In Southern blotting experiments with DNAs of somatic cell hybrids that contained segments of chromosome 11, we were able to assign the T3-delta gene to the distal portion of the long arm of human chromosome 11 (11q23-11qter). By use of a newly developed cDNA clone (pPEM-T3 delta) that codes for the murine T3-delta chain, the mouse T3-delta gene was mapped on chromosome 9. The importance of the T3-delta map position and its relationship to the other genes on the long arm of human chromosome 11 and to those on mouse chromosome 9 is discussed.