Abstract The present studies establish the enzymatic methylation of lysine residues in certain proteins with S-adenosylmethionine as the methyl donor. Ribosomal proteins of the aquatic fungus Blastocladiella emersonii and basic proteins of chromatin from Ehrlich ascites carcinoma cell nuclei were shown to act as methyl acceptors. Crude protein preparations from Salmonella typhimurium were also able to accept methyl groups. In each case, only lysine residues in protein were methylated to yield e-N-methyllysine. The product of this reaction proved to be identical with authentic e-N-methyllysine by paper electrophoresis and column chromatography.