In case of accidental radiation exposure or a nuclear incident, physical dosimetry is not always complete. Therefore, it is important to develop tools that allow dose estimates and determination that are based on biological markers of radiation exposure. Exposure to ionizing radiation triggers a large-scale activation of specific DNA signaling and repair mechanisms. This includes the phosphorylation of γH2AX in the vicinity of a double-strand break (DSB). A DNA DSB is a cytotoxic form of DNA damage, and if not correctly repaired can initiate genomic instability, chromosome aberrations, mutations or apoptosis. Measurements of DNA DSBs and their subsequent repair after in vitro irradiation has been suggested to be of potential use to monitor cellular responses. The bone marrow and the blood are known to be the most radiosensitive tissues of the human body and can therefore be of particular importance to find radiation-induced biological markers. In the present study, changes in H2AX phosphorylation and apoptosis of irradiated human peripheral blood mononuclear cells (PBMCs) were analyzed.