Micrococcal nuclease catalyzes cleavage of both DNA and RNA to yield 3'-nucleotides. It exhibits exo- and endo-5'-phosphodiesterase activities. The enzyme catalyzes preferential endohydrolysis of the RNA and DNA at sites rich in adenylate or uridylate and deoxyadenylate or thymidylate. The enzyme has a molecular weight of 16,807 daltons and is calcium dependent. The pH optimum is 9.2 but varies depending upon the concentration of ionized calcium present.