The supernatant fraction of liver homogenate brings about the methylation of glycine by S-adenosylmethionine. The secondary amine formed in the reaction appears to be sarcosine. This conclusion is based on (I) the stoichiometry of the reaction, and (II) radioautography of chromatograms of the incubation products. The enzyme, glycine N-methyltransferase, was purified 80-fold, and certain of its characteristics are reported. The possible role of the enzyme in the biological oxidation of the methyl group of methionine is discussed.