A nuclease has been purified about 100-fold from ammonium chloride wash of rye germ ribosomes. The enzyme was electrophoretically homogeneous. Its M, was 20,000 and pl 4.8. The neclease hydrolyzed endonuclelytically DNA and RNA and was accompanied by 3-nucleotidase activity. The enzyme degraded RNA to oligonucleotides with a phosphomonoester bond at position 5', and both denatured and native DNA to 5'-OH and 3'-phosphate-terminated fragments. Zinc ions and 2-mercaptoethanol stimulated deoxyribonucleolytic activity. EDTA, polyamines and heparin had only little or no effect. The enzyme is a glycoprotein containing 28% of carbohydrate which consists of fucose, mannose and glucosamine. The nuclease isolated is classified as nuclease I.