Purpose : The impact of apoptosis on radiation-induced eradication of clonogenic tumour cells is uncertain. The aim was to analyse the relationship of different functional stages during the apoptotic process to cell death and clonogenic cell eradication. Materials and methods : Apoptosis in Jurkat T-cells was studied by morphology, light scatter and caspase activation. Mitochondrial integrity was determined by the mitochondrial membrane potential (ΔΨ m) . Cell death was quantified using propidium iodide exclusion. Clonogenic cell death was determined using a dilution survival assay. The influence of Bcl-2 was tested using a Bcl-2 transfected Jurkat clone. Results : Irradiation induced profound apoptosis within 48 h associated with caspase activation and breakdown of ΔΨm. Inhibition of caspases abrogated the apoptotic morphology with no influence on breakdown of ΔΨm and survival. Over-expression of Bcl-2 abrogated all hallmarks of apoptosis; delayed cell death, however, had no influence on clonogenic survival after irradiation. Conclusion : Based on Bcl-2 as a positional marker, radiation-induced apoptosis can be divided into two stages: the initiation/decision phase, characterized by a breakdown of the mitochondrial membrane potential, and the execution phase, characterized by caspase activation. The execution phase had no influence on survival, whereas the initiation/decision phase controls immediate survival. However, abrogation of both phases did not influence radiation sensitivity.