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IT is well known that when body cells are seriously injured, the blood and urine of animals are able to degrade deoxyribonucleic acid, in a way closely resembling the action of deoxyribonucleases I and II. In this communication we therefore refer to these activities as the effect of deoxyribonucleases. Several explanations of this phenomenon are possible. Two of them provided the background for this investigation: the (a) the enzymes are extruded from damaged cells, (b) with the release of the cell contents a reaction is started which ultimately leads to a rise in deoxyribonuclease activity. The facts in favour of this point of view were given by Kowlessar and Altman1. They found rising deoxyribonuclease activities following irradiation of the whole body. In rats the maximum activity in the body fluids of deoxyribonuclease II is found on the first, and of deoxyribonuclease I on the sixth day, following exposure to X-rays. This makes it unlikely that one mechanism would be responsible for both maxima.