Purification and properties of a 3''-phosphoryl former endodeoxyribonuclease from eggs of Asterias forbesi.  A DNA endonuclease has been purified from eggs of Asterias forbesi by a simple four-step-purification procedure. The purified enzyme is at least 96% pure and is free of phosphatase, phosphodiesterase, and RNase. It has a pH optimum of 6.5 and does not require divalent cations. The enzyme produces 3''-phosphoryl and 5''-hydroxyl end groups. The products of exhaustive hydrolysis can be grouped in two fractions. The first fraction, 40%, contains a small amount of mononucleotides and di-, tri-, tetra-, penta-, and hexanucleo-tides. The second fraction, 60%, contains oligonucleotides larger than hexanucleotides.