Background: Following human radiation exposure in hospital or accidents, dose assessments are of prime importance in radiation accidents. These issues are of continuing importance with respect to socioeconomic policy relating to the industrial and medical uses of ionizing radiation, and also for risk assessment among people who are occupationally exposed to low and/or high linear energy transfer (LET) radiation, such as astronauts, pilots, stewardesses, nuclear power plant workers, and victims of radiation accidents. Materials and Methods: In this study, an assay for assessing radiation dose based on the induction of apoptosis in human T-lymphocytes was done to examine T-lymphocyte cells isolated from the fresh blood of 16volunteers, cultured and exposed to gamma rays. Radiation-induced apoptosis (RIA) was assessed by flow cytometric identification of cells displaying apoptosis-associated DNA condensation. Results: Dose-response experiments showed that at 2Gy dose level of radiotherapy programs, the RIA frequency was significantly above control. Apoptotic levels significantly depend on the dose of radiation rather than the donor. Conclusion: The results demonstrate the potential use of this assay as a biological indicator of radiation toxicity, optimizing patient dose in radiotherapy and biological dosimetry process.