The preferential occurrence of immature T-cell receptor (TCR) δ rearrangements (i.e. incomplete Dδ2-Dδ3 and Vδ2-Dδ3) in B-cell precursor acute lymphoblastic leukaemia (BCP ALL) and of predominantly mature rearrangements (incomplete Dδ2-Jδ1, complete Vδ1, Vδ2, Vδ3 to Jδ1) in T-lineage ALL prompted us to establish two separate multiplex PCR systems for the identification of clonal TCRδ rearrangements. PCR products of the expected size for the specific rearrangements were detectable from a dilution of 100–1000 clonal cells in 150 000 polyclonal cells. Both multiplex PCR systems were used to analyse samples from 86 childhood BCP ALLs and 30 T-lineage ALLs. The results of the multiplex PCRs were controlled by standard PCR analyses for the individual rearrangements and Southern blots, which were identical. Only immature TCRδ rearrangements were detected in BCP ALL (59%), whereas no rearrangement was found in the remaining BCP leukaemias, thus confirming the exclusive presence of immature TCRδ rearrangements in B-lineage cells. 50% of the T-lineage ALLs contained mature rearrangements, but no immature rearrangements were found. These two multiplex PCR techniques appear to be reliable and fast aids in the analysis of clonal TCRδ rearrangements in ALL.