The prolonged infection of a normally lytic virus, vesicular stomatitis vrus (VSV) in mouse L cells was initiated in a novel way by interferon treatment of the cells before virus challenge. The mechanism of maintenance of this persistent infection was due to several factors. The major factors were the development of temperature-sensitive small plaque mutants and the production of endogenous interferon. No evidence for defective interfering VSV particles was uncovered. The L cells employed in these studies were also unable to amplify exogenously produced DI particles of VSV.