The binding of colloidal iron hydroxide (CI) and ruthenium red (RR) to the plasma membrane of frog ependymal astrocytes was examined by electron microscopy. Positively charged CI and RR bind to the external surface of the plasma membrane of all parts of the ependymal astrocyte. Prior treatment with neuraminidase markedly reduces the number of bound CI particles, suggesting that the sialic acid of carbohydrates associated with the cell surface is responsible for much of the CI binding. Comparable observations were made on mouse ependymal cells. These findings indicate that an anionic, carbohydrate-rich cell coat occurs on the plasma membrane of amphibian ependymal astrocytes and mammalian ependymal cells. This cell coat may be related to transport, barrier, or receptive functions of ependymal cells.