The avian pathogen Newcastle disease virus (NDV) is a member of the Paramyxoviridae, a family which includes Sendai virus, simian virus 5 (SV5) and the human virus parainfluenza 3 (para-3). These enveloped viruses contain a single-stranded RNA genome of negative sense, 15-18 kb in length, encased in a helical nucleocapsid. Six major messenger RNA species are transcribed sequentially by the virion-associated RNA polymerase. They are capped, methylated, and polydenylated by virion proteins (Huang et a/., 1971; Colonno and Stone, 1976). Organization of the NDV genome has been investigated indirectly by both uv-inactivation mapping and by the analysis of polycistronic transcripts (Collins et a/., 1980; Wilde and Morrison, 1984). Both studies indicate that the gene order is identical to that of Sendai virus, which has been determined to be 3’-NP-P-M-F-HN-L-5’ by nucleotide sequence analysis of genomic clones (Blumberg et a/., 1984, 1985a,b). NP and the phosphoprotein P are components of the viral nucleocapsid, L is an RNAdependent RNA polymerase, and M is a protein of undetermined function associated with the inner surface of the viral membrane (reviewed in Kingsbury, 1985). Paramyxovirus glycoproteins are functionally similar to the well-studied surface glycoproteins of influenza (Ward, 1981; Colman and Ward, 1985). They possess cell fusion, hemagglutination and neuraminidase activities (Hsu et a/., 1979; Scheid and Choppin, 1974; Merz et a/., 1981), and constitute the spikes which protrude from the viral envelope (Choppin and Compans, 1975). The F glycoprotein, which is responsible for the paramyxovirus’ ability to fuse with the host cell membrane, is activated by a proteolytic cleavage event (Scheid and