Temperature-sensitive mutants isolated from hamster and canine cell lines persistently infected with Newcastle disease virus.

Evidence is presented which confirms that temperature-sensitive (ts) mutants with an RNA- phenotype are spontaneously selected in persistent infection of cell lines with Newcastle disease virus. Persistently infected BHK-21 cells, maintained since 1973, produce no interferon and are completely susceptible to vesicular stomatitis virus. Persistent infection of a canine kidney cell line (MDCK) terminated with destruction of all cells at about 100 days. Even under these conditions, a high proportion (33%) of RNA- temperature-sensitive mutants was present in the virus population 60 days after the infection was initiated.