In vitro X-irradiation of human peripheral blood lymphocytes increased the frequencies of fluorodeoxyuridine-induced fragile sites in a dose-related manner. However, the cells from 30 atomic bomb survivors exposed to either high or low radiation doses 47 years earlier showed no demonstrable difference in fragile site expression, indicating that fragile site induction was ephemeral in nature. When fragile sites were analyzed on the basis of tobacco smoking habits, an elevated number was observed in the smokers. The results confirm that fragile sites can be affected by recent exposure to exogenous agents, but the effect is probably of limited duration, based on the atomic bomb survivor experience.