A study has been made of the intrinsic radiosensitivity of peripheral blood lymphocytes from normal individuals. Cell survival following in vitro high (1.55 Gy min-1) and low (0.0098 Gy min-1) dose-rate irradiation was obtained for single lymphocyte samples from three individuals in six experiments. Despite wide interexperimental variability the ranking of intrinsic radiosensitivity for these individuals was reproducible. Further studies were carried out using low dose-rate irradiation only, on: (1) single lymphocyte samples from 18 people; (2) 14 samples taken on different occasions from one of these donors over a period of 5 months; and (3) a large store of lymphocytes from another individual. The latter was assayed in every experiment and served as an internal standard to which results could be normalized. In comparison with uncorrected values, the normalized results for the 14 samples from the single individual showed a reduction in the spread of data while those for the 18 different donors were unchanged. An analysis of variance was carried out on a larger data set of 38 measurements which included repeat assays on several of the samples (two samples from the multiply-sampled individual and five samples from other donors were assayed twice). In comparison with the uncorrected values, the normalized results showed a reduction in experimental variability such that the coefficient of variation (CV) for surviving fraction at 4 Gy for the multiple samples from a single individual decreased from 41 to 19%. In contrast, there was little change in the variation between 18 individuals with CVs of 56 and 62%, respectively, for uncorrected and normalized results. These data suggest that the use of an internal standard for the determination of the intrinsic radiosensitivity of individuals using peripheral blood lymphocytes should aid the development and evaluation of predictive tests for the radiotherapy of cancer.