Molecular analysis of somatic cell hybrids derived from T cells carrying at (7; 14)(q35; q32) chromosomal translocation from a patient with ataxia telangiectasia and T cell leukemia indicates that the breakpoint on chromosome 14 is proximal to the IgH locus and to the D14Sl locus, while the breakpoint on chromosome 7 involves the T cell receptor p chain locus immediately 5’to Jf11, 5 on chromosome 7. The separation of VP and Co observed in somatic cell hybrids defined the orientation of the T cell receptor b chain locus on chromosome 7 where the VP genes are centromeric and the Co genes are telomeric. A novel chromosomal alteration, undetected cytogenetically, was revealed as being an inversion with duplication of the distal band of chromosome 14q32. The importance of the 14q32 region in the leukemogenic process is discussed.