The health effects of low-dose radiation exposure are of public concern. Although molecular events in the cellular response to high-dose-rate radiation exposure have been fully investigated, effects of long-term exposure to extremely low-dose-rate radiation remain unclear. Protein expression was analyzed by two-dimensional electrophoresis in livers from mice irradiated for 485 days (22 hr/day) at low-dose-rates of 0.032 microGy/min, 0.65 microGy/min and 13 microGy/min (total doses of 21 mGy, 420 mGy and 8000 mGy, respectively). One of the proteins that showed marked changes in expression was identified as rhodanese (thiosulfate sulfurtransferase). Rhodanese expression was increased after irradiation at 0.65 microGy/min and 13 microGy/min, while its expression was not changed at 0.032 microGy/min. Rhodanese is a detoxification enzyme, probably related to the regulation of antioxidative function. However, antioxidative proteins, such as superoxide dismutase (SOD)1 (also known as Cu,Zn-SOD) and SOD2 (also known as Mn-SOD), which can be induced by high-dose-rate radiation, were not induced at any low-dose-rates tested. These findings indicate that rhodanese is a novel protein induced by low-dose-rate radiation, and further analysis could provide insight into the effects of extremely low-dose-rate radiation exposure.