The genetic sequence of the third complementarity determining region (CDR III) of the immunoglobulin heavy chain (IgH) gene was analysed in 55 rearranged alleles from 36 children presenting with B-lineage acute lymphoblastic leukaemia (ALL). This confirmed the unique nature of these rearrangements. However, contrary to the hypothesis that the CDR III is produced by a random process of rearrangement, biased utilisation of diversity (D) segments and of joining (J) regions 4, 5 and 6 was demonstrated. Moreover, preferred sequence boundaries were seen in J regions 1 to 5 and were suggested at the 3'-end of certain D regions, notably D21/9 and DK1. Similar patterns of rearrangement have been noted in normal B-lymphocyte clones. Together with relatively limited N nucleotide addition, these factors may restrict the potential sequence variability at the D-N-J junction. The occurrence of clonal progression by secondary gene rearrangements, such as V-V replacement, favours the use of this site when designing clone-specific oligonucleotide probes for use in monitoring minimal residual disease (MRD). In cases where biased features of D-J rearrangement are shared by both the leukaemic and normal B-lymphocyte clones this could reduce the sensitivity of these probes in detecting low levels of residual disease.