Cockayne's syndrome (CS) is a rare autosomal recessive genetic disease characterized by mental and physical retardation, microcephaly, dwarfism, retinitis pigmentosa and a hypersensitivity to sunlight. Cells originating from patients also exhibit, in vitro, a hypersensitivity to UV radiation. Using a colony assay in vitro, we studied the sensitivity of 5 CS cell strains (GM739, BOR, CS697, CS698 and KA) and two normal ones (HF19 and GP) to UV- and gamma-irradiation. The 5 CS strains appear to be UV-hypersensitive but the sensitivity varies widely from one strain to another. Hypersensitivity to gamma-rays has been reported for 4 out of the 5 CS cell strains investigated. However, these CS cell strains are less sensitive to gamma-rays than are ataxia telangiectasia cells. The KA cell strain exhibits a normal response to gamma-irradiation. Repair of potentially lethal damage (PLD) after UV- and gamma-irradiation was investigated by using unfed plateau-cell cultures. Under these conditions, control cells show a great capacity to repair PLD (10- to 30-fold survival increase at 1% survival level). The two CS strains (GM739 and BOR), which are hypersensitive to both UV- and gamma-irradiation, exhibit no or only little PLD repair after treatment. In contrast, the normal response of KA cells to gamma-rays is associated with a normal PLD repair capability. This latter cell strain exhibits an intermediate sensitivity to UV and shows an intermediate PLD repair capacity. The response of CS cell strains after gamma-irradiation suggests a genetic heterogeneity. Three complementation groups are described in CS cells when dealing with UV radiosensitivity. However, variations in gamma-ray sensitivity are reported for cells within the same UV complementation group.