This study was intended to test for a possible early selective loss of relatively radiosensitive individuals from those atomic bomb survivors exposed to high doses using an in vitro X-ray dose-survival assay of peripheral blood lymphocytes. The assay was reasonably reproducible since the coefficient of variation (CV) was 8.2% for the mean D10 (the dose required to kill 90% of cells) of 3.39 Gy after 15 repeat tests for one control donor during the study period. The CV for single tests for 201 survivors was essentially the same, i.e., 7.7% with a mean D10 of 3.37 Gy, indicating very little heterogeneity of lymphocyte radiosensitivity among individuals. Linear regression analysis of D10 on the DS86 dose showed no evidence for the consistent change in average D10 values among the survivors exposed to high doses. The results might imply that the G0 lymphocytes do not express full variations in radiosensitivity and may not be suitable for quantitative measurements of relative radiosensitivity. Alternatively, the very small variation in lymphocyte radiosensitivity may be real and detection of the rare individuals with altered radiosensitivity may require much larger-scale testing. Therefore, no conclusive answer to the question of population bias was provided for the survivors.