DNA from transplantable hepatocellular carcinoma (THC) 252 has recently been found to have a lower 5-methylcytosine content than DNA from normal or regenerating rat liver. We have determined that DNA methylase, purified 200-fold from nuclei of regenerating rat liver, can add more methyl groups to THC 252 DNA than to DNA from normal or regenerating rat liver. Furthermore, a similarly purified DNA methylase from THC 252 was found to methylate THC 252 DNA at a higher rate than it methylated DNA from normal or regenerating liver. The larger number of unmethylated sites in THC 252 DNA was not due to a deficiency of DNA methylase since the level of methylase activity of nuclear extracts from THC 252 was 2.7 times that of normal liver and 1.5 times that of regenerating liver. Methylases from these three sources had similar rats of reaction with different DNA substrates. These findings suggest that the hypomethylation of THC 252 DNA is not due to decreased methylase activity or to altered enzyme specificity.