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Following non-lethal anoxic X-irradiation the bacterium Micrococcus radiodurans suffers a delay in deoxyribonucleic acid ( DNA ) synthesis and during this delay components are released from the cellular DNA into the cytoplasm and into the surrounding culture medium. These events are subsequent to the radiochemical damage suffered by the macromolecular structure of the DNA which can be detected by physico-chemical means (Dean, Feldschreiber & Lett 1966). The rate at which material is released (or excised) from the DNA is inde­pendent of the X-ray dose and the kinetics of the process suggest that an enzyme is effecting the majority of the release and that it is present in limiting amounts. Studies of the excision products indicate that the enzyme is an exonuclease.