The alkaline and acid DNAse and RNAse were histochemically investigated in the tumors of the human central nervous system. In most of the malignant tumors, none of the four types of nucleases manifested any activity, whereas in the benign tumors this activity was similar to that of those normal tissues from which they originated. The necrotic areas of malignant tumors had a peripheral zone in which the activity of the nucleases reappeared. This reappearance was probably due to the splitting of the nuclease‐inhibitor complex. The significance of nucleases in malignancy and their possible role in the protection of the genetic stability of the cell are briefly discussed.