Nuclear dimensions in mammalian cells appear as a determining factor of chromatin organization and cellular radiosensitivity. Most radioresistant interphase cells have a nuclear volume (Vn) of 75 to 2700 micron 3, that could allow both the topological organization of chromatin as loops attached to the inner surface of the nuclear envelope and the unfolding of condensed chromatin within the topological constraints existing along the DNA molecule. In contrast, the radiosensitive small lymphocytes, with Vn values of 20 to 65 micron 3, seem to comprise significant amounts of highly condensed chromatin and dispose of an uncompleted topological organization of DNA, which may cause their incapacity to perform replication and transcription of DNA as well as the repair of radiation damage at a cell level. The indications are that radiosensitivity (1/D37) of animal cells, containing a similar quantity of DNA, should be directly proportional to 1/nuclear volume (1/Vn). However, DNA is unevenly distributed within the nuclear space, according to a partial ordering of interphase chromosomes; and it appears that radiosensitivity increases in zones of high DNA or chromatin density.