Bcl-XL, a member of the Bcl-2-related anti-apoptosis protein family, antagonizes a diverse range of apoptosis-inducing stimuli by preventing mitochondrial permeability transition, release of apoptogenic factors including cytochrome C, and caspase activation. We have tested the hypothesis that the susceptibility of Bcl-XL-expressing leukaemic cells to apoptosis induced by VP16 (etoposide) can be enhanced by pharmacological downregulation of Bcl-XL in vivo. Two subcutaneous xenograft models of B-cell leukaemia-employing SEMK-2 and BV173 cell lines were established in severe combined immunodeficient/non-obese diabetic mice followed by 14 d of continuous subcutaneous administration of Bcl-XL-specific second generation oligonucleotides ISIS 16009 or ISIS 15999. Tumours were disaggregated, enabling investigation of Bcl-XL expression and apoptosis susceptibility at single-cell resolution using cytofluorimetry. Marked sequence-specific reduction of Bcl-XL was associated with sequence-specific enhancement of VP16-induced mitochondrial permeability transition, caspase-3 activation and loss of membrane asymmetry. A negative correlation between Bcl-XL expression and apoptosis susceptibility was observed, together with a positive correlation with respect to a reduced redox state. Bcl-XL downregulation reduces the threshold for VP16-induced apoptosis by potentiating mitochondrial dysfunction and its sequelae, and therefore presents a novel therapeutic strategy for reversing chemoresistance.