Tween-ether treatment of rubella virus, which has no effect on the antigenic and electrophoretic properties of the two envelope glycoproteins, destroys infectivity and enhances haemagglutinating activity. Trypsin treatment alters the electrophoretic pattern of the envelope glycoproteins so that the VPI peak is no longer evident and the VPII peak is reduced. At the same time, both the properties of haemagglutination and infectivity are inactivated but the capacity to combine with neutralizing antibody is retained, which suggests that VPII may be responsible for inducing the production of neutralizing antibody.