T cell receptor delta chain (TCR delta) gene rearrangements were studied by Southern blot analysis in 36 patients with common acute lymphoblastic leukaemia, including 14 adults and 22 children. The majority of patients (68%) had either a rearrangement or deletion of one or more TCR delta genes. The most frequent rearrangement involved a partial recombination of V delta 2 to D delta 3 (55%). D delta 2-D delta 3 rearrangements were present in five patients (14%). To investigate the TCR delta rearrangement as a tumour marker in minimal residual disease studies, presentation samples from 18 patients were amplified by PCR and directly sequenced. Although the size of the V delta 2-D delta 3 junction varied by only 40 bp, sequence analysis showed extensive diversity. This was derived from four factors: deletion of the 5' end of D delta 3 gene (15/18) and 3' end of V delta 2 gene (16/18); the presence of D delta 2 sequences (6/18); insertion of N nucleotides (15/18); association of P nucleotides with intact V delta 2 and D delta 3 genes (5/18). N nucleotides were the major feature, contributing to 75% of the junction. D delta 1 sequences were not involved. Twenty base oligonucleotide probes, constructed from the junctional sequences, were capable of detecting residual tumour cells at the 10(-4) sensitivity level. Cross hybridization studies confirmed the probes to be clone specific. Longitudinal studies on patients undergoing treatment were capable of detecting tumour in remission samples.