Mutation induction at the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus has been studied in three human bladder tumour cell lines of varying radiosensitivity. U1-S40b, a radiosensitive mutant clone of MGH-U1, has been previously reported to show no difference in split-dose recovery or low dose-rate sparing, but to have an impaired repair fidelity when compared to its parent line. In this paper we have shown that U1-S40b is less mutable at the hprt locus at a similar level of survival. This may represent an increased incidence of severe or non-repairable lesions, making hprt- mutants poorly recoverable in U1-S40b when compared to MGH-U1. No difference was seen in mutation induction between MGH-U1 and RT112, another human bladder tumour cell line of similar radiosensitivity to MGH-U1.