The soluble chromatin in mouse thymus after irradiation with differnt doses has been studied in detail. The amount of soluble chromatin increases at 2 hrs after a dose of 800 rads, reaches maximum at 15 hrs, and after 24 hrs goes down gradually. These changes are consistent well with the dying and scavenging process of cells in thymus.,At 2, 4 and 6 hrs after irradiation, a linear relationship between the amount of soluble chromatin and the doses has been found within the range of 50~800 rads.,The electrophoretic pattern of the polydeoxyribonucleotide(PDN)of soluble chromatin shows typical bands of internucleosome degradation when the irradiation dose reaches a definite level, but further increase of the dose does not cause further degradation of PDN. It seems that the increment of soluble chromatin at higher doses may be related to the higher mortality of cells.