3-METHYL HISTIDINE is a normal amino-acid component of myosin from adult muscle1 and of actin1,2. But whereas 3-methyl histidine occurs in actin isolated from adult and foetal muscle it is absent from myosin isolated from skeletal muscle of the foetal rabbit. After birth the amount of this amino-acid in myosin from rabbit skeletal muscle increases until it reaches the adult value3. This implies that development in muscle is accompanied by the methylation of histidine residues of myosin. Nothing is known of the mechanism of this process and, although it has not been directly demonstrated that the methyl group of methionine is a precursor of the methyl group of 3-methyl histidine found in actin, the results of Asatoor and Armstrong2, obtained with whole animals, are compatible with such a mechanism. We have studied in vitro the methylation of muscle proteins with S-adenosyl methionine labelled with 14C in the methyl group in an attempt to elucidate the mechanism of formation of the 3-methyl histidine present in myosin and actin.