The nuclei and the fraction of chromatin peripheral layer bound to the nuclear membrane were isolated from blood lymphocytes of healthy and chronic lympholeukosis (CLL)-stricken cows. The isolation procedure included treatment of nucleic with DNase I with subsequent extraction of chromatin by low ionic strength salt solutions and purification of nuclear membranes by sucrose density gradient centrifugation. The blood lymphocyte nuclei from cows suffering from CLL differ from nuclei of healthy animals by a higher degree of chromatin condensation and a lower RNA content. Amount of DNA in the peripheral chromatin layer in lymphocyte nuclei of cows suffering from CLL is two times as high as that in healthy animals. The GC-content in the peripheral chromatin layer DNA from lymphocyte nuclei of healthy and CLL-stricken animals does not differ from that of total nuclear DNA (about 45 mol.%). Both in healthy and CLL-stricken animals, the 5-methylcytosine content in the chromatin peripheral layer DNA is by about 20% less than in the total nuclear DNA of healthy cows. In the chromatin peripheral layer DNA of blood lymphocytes from CLL-stricken animals the methylation level is unaffected. Therefore, a certain decrease in the total nuclear DNA methylation in CLL-stricken animals observed is due to methylation changes in DNA which is not bound to the nuclear membrane.