Most of the information on the structure, function and replication of paramyxoviruses has been obtained from studies on Newcastle disease virus (NDV), simian virus 5 (SV5), and Sendal virus (see reviews by Kingsbury, 1973; Choppin and Compans, 1975; Rott and Klenk, 1976). The virion consists of a helical ribonucleoprotein, the nucleocapsid, which is surrounded by an envelope (Fig.l). The nucleocapsid is composed of the single stranded RNA genome and a carbohydrate-free polypetide subunit, the NP protein. The envelope contains lipid which is derived from the plasma membrane of the host cell (Klenk and Choppin, 1969) and is present in the form of a bilayer (Landsberger et al., 1971). Associated with the outer side of the bilayer are spikes which are composed of glycoproteins (Klenk, et al., 1970; Chen, et al., 1971), the inner side is coated by the carbohydrate-free M protein. There are 2 types of spikes: one type consists of glycoprotein HN which has hemagglutinating and neuraminidase activity (Scheid et al., 1972; Scheid and Choppin, 1973; Seto et al., 1973;Tozawa, et al., 1973; Shimizu, et al., 1974), and the other one consists of glycoprotein F which induces cell fusion and hemolysis (Homma and Ohuchi, 1973; Scheid and Choppin, 1974; Seto, et al., 1974). It is generally agreed that these activities reflect the essential roles which both