A number of facts suggest that chromatin autodigestion, occurring in the early phase of apoptosis, is carried out by an enzymatic system, composed of an endonuclease and a protease, which yields oligonucleosomic chromatin fragments. Though this enzymatic system appears to be present in most mammalian cell nuclei, radiation-induced apoptosis takes place, with a high frequency, only in cell populations having less well-developed nuclear matrices, such as lymphoid cells. Moreover, apoptosis seems to occur in a different manner in cells with less well-developed nuclear matrices (radiosensitive cells) compared with cells that contain dense nuclear matrices (radioresistant cells). Thus, dying lymphocytes progressively release their degraded chromatin from nuclei, without displaying the cellular budding and formation of apoptotic bodies. Nevertheless, apoptosis remains the main cause of cell death and cell depletion in irradiated lymphoid tissues. In contrast, the process of cellular budding and formation of apoptotic bodies appears to be specific for cells having well-developed nuclear matrix, such as those from small intestine and liver. However, in these tissues the frequency of apoptosis is relatively low and cannot be considered as the main cause of radiation-induced tissue involution.