The radiobiological effects of thiol modifier BSO on retinoblastoma were studies using cultured retinoblasdtoma cell lines Y-79 and So-Rb 50 and retinoblastoma bearing nude mouse. The preliminary results shown that BSO can deplete intracellular GSH contents of retinoblastoma cells in vitro and vivo. In vitro data demonstrated that lowaand nontoxic concentration BSO increased the retinoblastoma cells radiosensitivity especially under the hypoxic condition.