Decrease in electrophoretic mobility of erythrocytes, increase in Km of erythrocyte membrane acetylcholinesterase and decrease in the binding constant of 8-anilino-1-naphthalene sulfonate to erythrocyte membranes demonstrate a decrease in surface charge density of bovine erythrocytes during in vivo aging. This phenomenon seems to be species-specific; it may be due to a diminution of the sialic acid content but may also be contributed by conformational changes of membrane proteins.