A chromosomal translocation t(11;14) (p15;q11) is described in a human acute T-cell leukaemia of immature phenotype (CD3-, CD4-, CD8-). The translocation occurs at a T-cell receptor joining J delta segment, 12 kb upstream of the constant C delta gene and 98 kb upstream of the C alpha gene at chromosome band 14q11. Nucleotide sequencing shows that both J delta and C delta are very conserved between mouse and man. The region of chromosome 11 involved in the translocation is transcriptionally active and produces a 4-kb mRNA. The DNA sequence at the chromosome 11 junction shows a perfect match to a recombinase signal sequence implying that this translocation occurred by recombinase error. The occurrence of the translocation breakpoint at the C delta locus, normally rearranged in immature T cells, and the structure of the translocation junctions suggests that the translocation occurred during an attempt at normal rearrangement of the J delta segment in an early thymocyte.