Four X-ray-sensitive mutants of CHO cells, described previously by Jeggo and Kemp (1983), showed enhanced sensitivity to both 60Co gamma-rays and 238Pu alpha-particles relative to the responses of the parent line. The enhanced response to a densely ionising radiation (alpha-particles) was less than that to X- or gamma-rays, suggesting that these mutants are deficient mainly in the repair of damage from relatively sparsely ionising radiation tracks. Plateau-phase cultures of the parental CHO cells showed considerable recovery upon irradiation with low-dose-rate gamma-rays, compared to irradiation at 'high' dose rates, but little or no recovery was seen for the mutants. Similarly, preliminary data on recovery during post-irradiation holding of plateau-phase cultures show that this process is also absent in the mutants. These responses have several similarities to those of cells from patients with the radiosensitive disorder ataxia telangiectasia (AT), and are discussed with reference to AT cells and other radiosensitive mutants.