SUMMARY

Monoclonal antibody to the haemagglutinin-neuraminidase or the fusion glycopro-tein 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. Monoclonal antibodies (MCA) have been used to analyse the mechanisms of virus neutralization and to immunoselect variants (see Oxford, 1982). The neutralization of Newcastle disease virus (NDV) by MCA 445 to the haemagglutinin-neuraminidase (HN-2) epitope and MCA 481 to fusion (F) protein was independent of haemagglutination inhibition (HI) whereas a weaker neutralization, which could be potentiated by complement, was associated with HI by MCAs 14, 32 and 86 to the HN-1 epitope (Russell et al., 1983b). In the present paper MCAs 14, 445 and 481 have been used to investigate virus penetration by the procedure of Massey & Schochetman (1981) who showed that preadsorbed mouse mammary tumour virus (MMTV) resisted neutralization by MCA. Preadsorbed NDV, in contrast, wa