Abstract Purified membrane glycoprotein was found to specifically bind to erythrocyte surfaces. (1) The glycoprotein showed over 80-fold greater binding to erythrocytes than did bovine serum albumin; (2) the binding of the glycoprotein was not significantly inhibited by the presence of other proteins even when added in 175-fold greater concentration; (3) the glycoprotein's binding showed both time and concentration dependence. Radiolabeled glycoprotein was re-isolated after binding to erythrocytes and, after sodium dodecyl sulfate-polyacrylamide gel electrophoresis, over 64% of the label was recovered from the glycoprotein bands. The glycoprotein preparation would bind to erythrocyte surfaces to the extent of 6–7% of that naturally occurring in the membranes.