The cytopathogenicity and production of Newcastle disease virus (NDV) strain Herts cultivated in chick embryo (CE), baby hamster kidney (BHK 21), HEp 2, MDBK and L929 cells was investigated. Infection at high multiplicities (1000 p.f.u./cell) induced cell fusion in cultures of all cell types within 3 hr after infection. Infection at low multiplicities (10 to 20 p.f.u./cell) produced extensive cell fusion in CE, BHK 21, HEp 2 and L929 cultures within 24 hr, but MDBK cells failed to fuse. The latter cells failed to show high levels of virus hemagglutinin at the cell surface or accumulation of virus products, but infective virus was released to significantly higher titers than from the cell types which fused. However, infected MDBK showed similar levels of virus induced cell damage to the plasma membrane, as measured by the release of lactate dehydrogenase, to HEp 2 and L929 cells which were susceptible to fusion. The morphology of the c.p.e. produced by strain Herts in the different cell types is described. The relationship of virus accumulation and virus release to the process of cell fusion is discussed.
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