It is established that chromatin denaturation in the cell composition under conditions approximating to equilibrium is a quasistatistical process. It is shown that thermal treatment of tissues in chromatin composition enables to reveal the thermostable block with the following parameters of denaturation Td = 89 degrees C, Td = 3.5 degrees C and Qd = (28 +/- 4) J/g DNA. It is discovered that thermostability of tumor chromatin increases compared with the normal one, and this value in case of leucosis fluctuates from 5 degrees C to 10 degrees C, but in the case of sarcomas and ascite hepatoma of rats and mice it fluctuates from 2 degrees to 5 degrees C.