Radiobiological studies in the naevoid basal cell carcinoma syndrome. The naevoid basal cell carcinoma syndrome is described in a 43-year-old male. The response of dividing fibroblasts from this patient to the lethal effects of ionizing radiation was indistinguishable from that of a normal control: mean inactivation dose (D) = 1.74 Gy and 1.68 Gy, respectively. However, these cells were defective in the repair of potentially lethal damage (D = 2.36 Gy and 3.26 Gy, respectively). The response to radiation of Go T lymphocytes derived from the patient was similar to that of the control. The frequency of mutant circulating T lymphocytes resistant to 6-thioguanine was 17 x 10(-6) which was elevated when compared with age-matched controls. These results suggest an abnormality in the response of the fibroblasts to ionizing radiation damage. 