Recent advances in neuro-immunology are beginning to elucidate several essential roles of the humoral immunity in both repair and pathogenesis of central nervous system diseases. Immunoglobulin M (IgM) was reported to accelerate the rate of remyelination in a demyelinating disease model, while intrathecal IgM synthesis was shown to predict a more worse disease course in a human demyelinating disease, multiple sclerosis. Molecular mechanisms for either of these IgM functions remain to be studied. Here we report that a recently-found Fc receptor for IgM, namely Fcalpha/muR, is expressed in oligodendrocytes, their precursor cells, and myelin. These expressions were confirmed by immunocytochemistry of cultured cells, Western blot analysis of myelin fractions, and also by immunohistochemistry of mouse forebrains at different ages. Our findings suggest a possible direct interaction between IgM and Fcalpha/muR expressed in oligodendrocytes and myelin.