Thymocytes are highly radiosensitive and show 'interphase death' within a few hours after low doses of irradiation. However, the mechanisms responsible for this type of death remain ill-defined. Separation of the dead thymocyte fraction from irradiated thymocyte suspensions by centrifugation on Percoll gradients provided homogeneous populations of dead cells suitable for detailed study. Using this method, radiation-induced interphase death of thymocytes was found to involve a sharp but transient increase in buoyant density, concomitant with the appearance of distinctive morphologic changes which included disappearance of microvilli and blistering of the cell surface. The chromatin in the dead cells had a molecular weight sufficiently low to resist sedimentation, and consisted of short oligonucleosome chains. We were unable to detect populations of cells intermediate between the dead and normal in the above characteristics. Interphase death thus involves a discrete, abrupt transition from the normal state and is not merely the consequence of progressive and degenerative changes. Furthermore, immediate cessation of development of interphase death by cycloheximide suggested a possible involvement of protein synthesis on this transition step.