The agent of measles (rubeola) has been assumed to be a paramyxovirus on the basis of its electron microscopic appearance, its size, its sensitivity to ether and detergents, and the presence of hemagglutinin in the viral envelope. Thus far, however, the one attribute common to myxoviruses, namely neuraminidase, has not been described for measles virus. We have demonstrated that specific neuraminidase activity is associated with measles viral hemagglutinin. The evidence suggests strongly that the enzyme is part of the viral envelope.