Several viruses agglutinate erythrocytes of various animal species. The components responsible for this phenomenon have been termed hemagglutinins (HA), which in many cases have been identified as glycoproteins present in virus envelopes. The HA titers have often been used as convenient indicators of the amounts of viruses. Recent studies on virus HA dealt not only with their structures and functions in virus replication but with the molecular mechanisms of their biosynthesis as well, applicable in general to membrane glycoproteins.