The human melanoma cell lines MM96L, A2058 and HT144 were examined for sensitivity to ionizing radiation and UVB radiation. HT144 demonstrated a significant increase in sensitivity to ionizing and UVB radiation compared with the MM96L and A2058 cells. Sensitivity to both agents was associated with susceptibility to apoptosis. Using a protein truncation assay, a mutation for the gene for ataxia telangiectasia (ATM) was identified in HT144 cells. This was confirmed to be a homozygous mutation by subsequent sequencing of the abnormal region. Protein truncation assay of the other two cell lines showed no abnormality. The results suggest that somatic mutation of the A-T gene may be important in determining tumour radiosensitivity.