A panel of nine monoclonal antibodies has been used to construct a functional inhibition profile of four antibody-binding sites previously delineated on the HN glycoprotein of Newcastle disease virus. Antibodies to all four sites are capable of neutralizing infectivity and inhibiting hemolysis and neuraminidase activity with fetuin. There is a good correlation between the fractions of infectivity and neuraminidase activity which survive antibody treatment. However, the inhibition of hemolysis is in the inverse relative order of the neutralization of infectivity. Antibodies to sites 1 and 2 are capable of inhibiting hemagglutination, whereas only antibodies to site 2 can inhibit neuraminidase activity on the smaller substrate N-acetyl neuraminlactose. This antigenically separates the hemagglutinin and neuraminidase sites on the HN glycoprotein. We used these functional inhibition studies to speculate about the location of the four antigenic sites relative to the hemagglutinin and neuraminidase sites.