The levels of deCMP deaminase and deCR deaminase have been studied in normal and neoplastic human tissues. High levels of deCMP deaminase have been found to occur in all the human malignant tumors studied in the present work, the increase of the levels of deCR deaminase is less marked. Therefore, the ratio deCR deaminase/deCMP deaminase is lower in malignant tumors than in benign tumors and in normal tissues.