T cell prolymphocytic leukaemia (T-PLL) is a chronic mature T cell malignancy with many random cytogenetic abnormalities. These imply that maintenance of genomic integrity is impaired. This is supported by the recent finding that the ataxia telangiectasia gene, ATM, which contributes to maintaining genomic integrity, is frequently mutated in this disease. To evaluate in T-PLL the role of other genes with comparable function, a fluorescence-based semi-automated assay was developed for BAT-25 and BAT-26. These markers contain sequences that are particularly unstable in cells with DNA mismatch repair defects. Application of the assay to 20 T-PLL cases found no evidence for such defects.