DNA repair was measured in an ionising radiation-sensitive mutant of a human bladder carcinoma cell line. No difference in the rate or extent of double-strand break rejoining was found using the techniques of neutral filter elution and pulsed-field gel electrophoresis. In contrast, significant differences in repair fidelity, measured by plasmid reconstitution, were found. The parent line had a repair fidelity of 84.7% compared with 58.9% for S40b (P = 0.0003). It is suggested that repair fidelity can be an important determinant of radiosensitivity in human tumour cells.