Myelin was isolated from rat sciatic nerve and its composition determined. Peripheral nerve myelin contained 21% protein and the polypeptide-chain components were analyzed on polyacrylamide gels in presence of sodium dodecylsulfate. Sciatic nerve myelin contained at least seven polypeptide components ranging in molecular weight from 50 000 to 16 000. The two major polypeptides had a molecular weight of 28 000 and 22 000 and they were shown to be glycoproteins as they also stained with periodic acid—Schiff reagent for carbohydrates.