The neutralizing characteristics of monoclonal antibodies directed to four antigenic sites on the hemagglutinin-neuraminidase glycoprotein of Newcastle disease virus were determined. Neutralization by each antibody resulted in a persistent fraction of nonneutralized virus which varied from 1 to 17% depending on the hemagglutinin-neuraminidase site recognized, but not on the antibody. The addition of antibodies to all four sites on the hemagglutinin-neuraminidase glycoprotein was required to give a level of neutralization comparable with that obtained with polyclonal mouse antiserum. The high persistent fractions were not due to viral aggregates, a high level of variants in the virus stock, the use of insufficient antibody, low antibody avidity, or an effect peculiar to the use of the chicken cells as host. The addition of rabbit anti-mouse immunoglobulin to the persistent fraction left by any of the antibodies resulted in a further reduction in infectivity, often by as much as two logs. Thus, some viral particles are capable of binding antibody while retaining their infectivity. The implications of these findings to the mechanism of neutralization are discussed.