The substrate specificity of micrococcal nuclease (EC 184.108.40.206.) has been studied. The enzyme recognises features of nucleotide composition, nucleotide sequence and tertiary structure of DNA. Kinetic analysis indicates that the rate of cleavage is 30 times greater at the 5' side of A or T than at G or C. Digestion of end-labelled linear DNA molecules of known sequence revealed that only a limited number of sites are cut, generating a highly specific pattern of fragments. The frequency of cleavage at each site has been determined and it may reflect the poor base overlap in the 5' T-A 3' stack as well as the length of contiguous A and T residues. The same sequence preferences are found when DNA is assembled into nucleosomes. Deoxyribonuclease 1 (EC 220.127.116.11.) recognises many of the same sequence features. Micrococcal nuclease also mimics nuclease S1 selectively cleaving an inverted repeat in supercoiled pBR322. The value of micrococcal nuclease as a "non-specific" enzymatic probe for studying nucleosome phasing is questioned.