An express method for measuring the level of in vitro DNA methylation in homogenates and nuclei from animal tissues as well as during initial steps of DNA methylase isolation and purification when methylase activity is low and hardly testable by other methods has been suggested. The method is based on the measuring the radioactivity incorporated in filter adsorbed DNA (acid-insoluble material) 3H-label from S-adenosile-L-methionine as a result of in vitro DNA methylation. The advantage of the method consists in the replacement of a long-duration repeated deproteinization procedure traditionally used by a relatively simple procedure (15 min incubation of the mixture at 80 degrees C with 10 volumes of the 8M urea, 5 mM EDTA, 5% n-butanol, 2% sodium dodecilsulfate, 1 M sodium chloride solution) and the absence of any loss of DNA. The method is fit for the fast serial assay of DNA methylase activity taking into consideration that about one third of the total acid-insoluble radioactivity is due to the radioactivity in 5-methylcytosine residues in DNA.