Radiation protection regulations have been established to reduce exposure of individuals to acceptable safe levels. These limits assume that people have similar responses to ionizing radiation and that there is no variation in individual radiation risk. The purpose of this research was to determine if apoptosis in lymphocytes can be used to assess individual sensitivity to ionizing radiation. Blood samples were taken from 54 males ranging in age from 19–85 years. Apoptosis was measured using modified flow cytometry based Annexin-FITC/7AAD and DiOC6/7AAD assays in different populations of lymphocytes (total mixed lymphocyte population, subset CD4+ or CD8+ lymphocytes) after exposure to in vitro doses of 0, 2, 4 or 8Gy (dose rate 0.1Gy/min). The variation in individual responses to radiation was large. The variation was the largest in the CD4+ lymphocyte subpopulation. Radiation-induced apoptosis decreased with age of donor demonstrating that as people age their lymphocytes may become relatively more resistant to radiation. This research shows that individuals have marked differences in their sensitivity to radiation and protection policies may someday need to be tailored for some individuals.