We describe a rapid assay to predict intrinsic radiosensitivity of normal tissues based on the radiation-induced cytotoxic response of a number of types of white blood cells. Twenty-four hours after irradiation, leukocytes were prepared for analysis by flow cytometry. Radiation-induced cytotoxicity was characterized by degradation of internucleosomal DNA, which results in a reduced G1/G0-phase DNA peak. Clear differences in the radiosensitivity of five of six different types of leukocytes were observed. Significant increases in cytotoxicity were observed even at 0.5 Gy, indicating the technique may be useful as a biological dosimeter after radiation accidents. Based on data from 12 healthy blood donors, significant inter-donor differences in radiosensitivity were observed after exposure to 2 or 8 Gy of high-dose-rate X rays. Inter-donor variation was significantly greater than intra-donor variation for both CD4 and CD8 T lymphocytes (P < 0.01), and the technique yielded adequate data to stratify donors based on the radiosensitivities of these two cell types alone, requiring less than 1 ml of blood. The assay is a general assay of early cytotoxic response to radiation-induced damage and should be useful when assaying the response to chemotherapy as well.