The DNA methylating system of cellular nuclei from intact or virus transformed chicken liver was studied. The presence of multiple forms of methylases different in hydrophobic properties and isoelectric focusing points has been proved. The isoelectrofocusing made it possible to differentiate between the enzymes methylating preferably nonmethylated in vitro or methylated in vivo DNA. The DNA-methylases pool contains both types of methylases (de novo and supporting ones) in intact cells and at neoplastic transformation, however, the specificity of methylation and level of several enzymes in transformed cells is changed in the direction of broad specificity and lower activity. The general level of methylase activity at viral transformation is by 18-20% lower, while the content of 5-methylcytosine in hepatoma cells DNA is twofold lower as compared with the content in the DNA of intact cells.