Purified reoviruses types 1, 2, and 3 were specifically inhibited in their hemagglutination of human erythrocytes by 250 micromoles or more of N-acetyl-D-glucosamine but not by over 20 other sugars tested. N-acetyl-D-glucosamine inhibited reovirus hemaglutination by binding to capsid virus; it did not attach to the erythrocytes. It is possible that reovirus hemagglutination involves union between N-acetyl-D-glucosamine on the surface of the red cell and the glycoprotein of the virus coat.