The comet assay, a sensitive method of measuring DNA strand breaks in individual cells, is also capable of identifying apoptotic cells which contain highly fragmented DNA. This method requires embedding cells in agarose, lysing cells to remove proteins, and providing a brief exposure to an electric field to allow broken pieces of DNA to migrate. TK6 human B lymphoblast cells undergo fragmentation which is dependent on both time after irradiation and radiation dose. While some TK6 cells undergo apoptosis within 2 h after irradiation, the fragmentation rate increases approximately 10 h after exposure to radiation doses of 2.5 to 15 Gy. Results confirm that apoptosis is a very rapid event since few cells with intermediate amounts of DNA damage were detected. The comet assay detected apoptotic TK6 cells much earlier than a flow cytometry method.