The 1 hour incubation of calf thymus nuclei at 37 degrees C leads to a proteolysis of the histones H1, H3 and H2B. Urea does not influence the histone degradation while 1.5 and 2.0 M NaCl lead to the proteolysis of the H2A histone. On this background, 2 M urea restrains the degradation of the H2A histone. It is assumed that hydrogen bonds are very important for the activity of the proteinases and its interaction with the H2A histone.