The DNA in chromatin of isolated thymus nuclei, and in two different preparations of deoxyribonucleoprotein extracted from chromatin, has been digested with DNase. By variation of the time of reaction and the concentration of enzyme, and by comparison of the action of the enzyme on (protein-free) DNA with its action on DNA complexed with protein in chromatin and deoxyribonucleoprotein, it was found that most, if not all, of the DNA is linked to protein. The linkage is loose, as shown by the fact that all the DNA is accessible to the enzyme; however, the wide variations in accessibility in a given complex show that the looseness of the DNA-protein link varies considerably. Accessibility of DNA in one type of deoxyribonucleoprotein preparation is greater than it is in chromatin, while in the other type it is less than in chromatin.