Extract of normal Escherichia coli CK cells methylated DNA of Cd phage in vitro in presence of 3H-S-adenosyl methionine, although in vivo methylation of viral DNA did not occur. Extract of cells, infected with Cd phage, also was unable to methylate the DNA in vitro due to de novo synthesis of the enzyme splitting S-adenosyl methionine to 5'-methyl thioadenosine and homoserine. The enzyme was not found in cells, infected with the phage in presence of chloramphenicol. The activity of the S-adenosyl methionine splitting enzyme was shown to be maximal between the fourth and fifth min of infection. This suggests that the enzyme is synthesized under control of the virus genome, it belongs to the early virus-specific proteins and inhibits the methylation of the phage DNA in vivo. In unpurified extracts of Escherichia coli CK the methylating activity was completely inhibited by 10(-5) M S-adenosyl homocysteine.