Abstract
Monoclonal antibody to the haemagglutininneuraminidase or the fusion glycoprotein of Newcastle disease virus blocked virus penetration which otherwise occurred within 90 s of the temperature being increased from 4 °C to 37 °C. These antibodies regularly immunoselected variant plaques only when incorporated into the agarose overlay. Variant viruses then formed plaques whereas the growth of non-neutralized normal virus was suppressed.