Myelin is a membrane characteristic of the nervous tissue and functions as an insulator to increase the velocity of the stimuli being transmitted between a nerve cell body and its target. Myelin isolated from human and bovine nervous tissue is composed of approximately 80% lipid and 20% protein, and 30% of the protein fraction constitutes myelin basic protein (MBP). MBP has an unusual amino acid at Res-107 as a mixture of NG-monomethylarginine and NG, N'G-dimethylarginine. The formation of these methylarginine derivatives is catalysed by one of the subtypes of protein methylase I, which specifically methylates Res-107 of this protein. Evidence is presented to demonstrate an involvement of this biological methylation in the integrity and maintenance of myelin.