Antibodies against myelin proteins were utilized in the analysis of total particulate material from the brains of the agnathan hagfish and lamprey. Immunoblotting revealed in both species the presence of bands at 50,000 dalton that reacted with anti-bovine PNS-P0 antibodies. Single bands of 34,000 dalton and 51,000 dalton were immunodetected with anti-trout CNS-36K antibodies in lamprey and hagfish, respectively. Antibodies against mammalian myelin basic protein (MBP) and proteolipid protein (PLP) were not recognized. In spite of the lack of multilayered myelin in agnatha, the presence of myelin-related integral proteins suggests that agnathan glial cells have already acquired the capacity to synthesize some proteins that are similar to typical myelin proteins. This represents a crucial evolutionary step towards myelination.