A DNAse was isolated from rat brain and purified 1100-fold using affinity chromatography on a column with DNA-agarose and chromatography on granulated hydroxyapatite. The electrophoretically pure Mg2+, Mn2+-dependent enzyme preparation hydrolyzes the denaturated DNA with a maximal activity at pH 8,4. The optimal terminal concentration of Mg2+ corresponds to the Mg/phosphorus molar ratio of the substrate is 1:2. For Mn2+ this correlation is 1:1. Using the immobilized substrate method, the exonuclease type of DNAse activity has been established. The enzyme activity depends on the state of its SH-groups; the reaction is inhibited by pCMB. The molecular weight of DNAase as determined by gel-filtration through Sephadex G-200 is equal to 60 000.