Mapping of cleavages inflicted by phosphodiesterase on two viral DNAs has been accomplished; PM2 DNA has 5 and phi X174 DNA RF form I has at least six susceptible sites. Except for one site on PM2 DNA all the established cleavages are located in the regions known to be rich in A plus T. However, other regions equally rich in A plus T are not cleaved. Therefore additional structural features are required to make the site susceptible to phosphodiesterase. Studies of enzymes belonging to the group of single-strand specific nucleases continue, using chromatin as a substrate. An early stage of the digestion leads to very large nucleoprotein fragments ranging from 1 to 10 million daltons. These fragments retain nucleosomal structure, and range from 80 to 8 nucleosomes.