1. A method is described for the isolation and purification of desoxyribonuclease from a 0.25 N sulfuric acid extract of beef pancreas. The activity of the enzyme is measured by a viscosimetric method using sodium desoxyribonucleate from calf thymus as substrate. 2. The enzyme is highly active, a measurable effect being obtained at concentrations of less than 0.01 microgram per cc. In highly dilute solution the enzyme is rapidly inactivated, and the use of a protective agent such as gelatin or peptone is necessary. 3. The purified material contains traces of a proteolytic enzyme, but displays no ribonuclease, lipase, or phosphatase activity. 4. The enzyme requires activation by magnesium or manganese ion, and citrate serves as a potent inhibitor of the magnesium-activated enzyme. 5. Its enzymatic activity is inhibited by the specific antibody present in the serum of rabbits immunized with enzyme protein.