A type II modification methylase (M BstVI) was partially purified from the thermophilic bacterium Bacillus stearothermophilus V. The methylase catalyses the transfer of methyl groups from S-adenosyl-L-methionine to unmodified double-stranded DNA. The product of methylation was identified by paper chromatography as N6-methyladenine. Since M BstVI protects DNA against cleavage by BstVI and XhoI restriction endonucleases, it follows that it methylates the adenine residue in the sequence 5'-C-T-C-G-A-G-3'.