THE major central nervous system (CNS) myelin proteolipid (PLP) is also expressed in the peripheral nervous system (PNS). This paper gives evidence that DM-20, an isoform of PLP, also occurs in rat sciatic nerves, where, in contrast to what is seen in CNS myelin, it predominates over PLP. This conclusion was reached on the basis of results obtained by immunoblot analysis of a crude proteolipid extract from adult peripheral nerve with two site-specific anti-proteolipid (PLP and DM-20) antibodies. This finding was further corroborated by characterization of the products obtained by Polymerase Chain Reaction (PCR) amplification of cDNAs synthesized from total RNA of 14-day-old sciatic nerves. The significance of the occurrence of these proteolipids in PNS remains obscure.