Optimum conditions were determined for solubilizing cell membrane receptors for binding measles virus (MV). Evidence for specific receptors was shown from the saturability of MV binding to intact Vero cells or African Green Monkey erythrocytes (MRBCs). Receptors, solubilized from Vero cells and MRBC with 1.5% octyl glucoside, inhibited MV attachment, infectivity, and hemagglutination activities. Extracts from chicken erythrocytes, which did not bind MV, were inactive in all assays. MV binding activity in Vero or MRBC extracts was stable to heating (100 degrees, 10 min) or neuraminidase treatment, but was inactivated by a range of proteases, including chymotrypsin, and bound to lentil and pea lectin agaroses, to indicate a glycoprotein component.