DNAase was isolated and purified from cell-free extracts of Proteus mirabilis by means of fractionation with ammonium sulfate and CM cellulose chromatography. The enzyme exhibited the endonuclease specificity with DNA as a substrate, split off the native DNA by the "single-strike" mechanism, had a pH optimum in alkaline zone, required Me2+ and was inhibited by tRNA. The highest specific activity and the specific rate of the enzyme synthesis were found at lag phase, before the exponential phase of the cell population growth. Microorganisms of Proteus and Salmonella genera had the highest enzymatic activity in cell-free extracts as compared with other enterobacteria. Possible biological functions of the enzyme are discussed.