MYELIN is the lipoprotein multimembrane that functions as an insulator preventing the flow of ion currents across the axonal membrane and facilitating the conduction of nerve impulses. It is synthesized by oligodendrocytes in the central nervous system at about the time of birth in mammals1. During the initial stages of myelination, several proteins are phosphorylated on tyrosine. Among these proteins, we identified Fyn tyrosine kinase, one of the non-receptor-type tyrosine kinases of the Src family. Here we report that Fyn tyrosine kinase is activated during the initial stages of myelination and that it is associated with the large myelin-associated glycoprotein (MAG), an adhesion molecule that has been implicated in myelinogenesis2. The Fyn-large MAG association requires amino-terminal domains of Fyn that include SH2 and SH3 (Src homology domains 2 and 3). Crosslinking of large MAG with antibody induces a rapid increase in the specific activity of Fyn kinase. These results indicate that Fyn participates in the initial events of myelination as a signalling molecule downstream of large MAG; indeed, we find that fyn-deficient mice exhibit impaired myelination.