Protein- and lipid-bound sialic acid was assayed in myelin fractions isolated by four different methods from rat brain homogenates. The extent to which non-myelin cellular membranes contaminate these fractions was assessed by electron microscopy and marker-enzyme assays. Small amounts of sialic acid found in the least contaminated myelin fractions are considered to be constituents of axonal and satellite cell plasma membranes known to be present. The data are discussed with reference to the ultrastructural appearance of myelin.
