Lymphocytes are very sensitive to radiation. Our aim was to test the possibility of detecting apoptosis in lymphocytes as a potential short-term biomarker of ionizing radiation exposure. Our in vitro data confirmed the dose-time-effect relationships involved in radiation-induced apoptosis. The detection of in vivo induction of apoptosis in circulating lymphocytes after exposure of animals to radiation appears to depend critically on the technique used to measure apoptosis. Among the different techniques we investigated, mitochondrial modification was the most appropriate; they allowed establishment of dose-time-effect relationships when animals were observed for 72 h. A model of in vitro phagocytosis of apoptotic lymphocytes by macrophages was developed to mimic clearance of apoptotic cells occurring in vivo. Together, our data show that mitochondrial labeling may make it possible to detect ex vivo radiation-induced apoptosis of lymphocytes before macrophage ingestion occurs. We propose the measurement of apoptosis in lymphocytes as a potential short-term biomarker of ionizing radiation exposure.