[ 3H]Proline derived radioactivity which is slowly transported in the goldfish optic nerve has been isolated in association with purified preparations of optic tectal myelin. Acrylamide gel analysis reveals that most of the slowly transported radioactivity is distributed among the high molecular weight mylein proteins with a major peak comigrating with a band ( ∼42,000daltons) which is prominent in goldfish brain myelin. A similar peak is not seen in gels of whole synaptosome protein. Relatively small but measurable amounts of radioactivity comigrate with the basic and proteolipid proteins. A low density myelin subfraction is 3–5-fold lower in specific radioactivity (disint./min/μg protein) than a corresponding high density fraction and shows particularly low specific radioactivities in the basic and proteolipid bands. The results are discussed with reference to the possible adherence of specific axonal membrane fragments as well as other potential sources of myelin label.