The course of persistent infection in Newcastle disease virus (NDV)-infected mouse L cells (LNDV) was analysed. The production of infectious intra- and extracellular virus was markedly reduced (less than 10(1) PFU/ml), 30% of the cells produced virus-specific antigen. Analysis of the synthesis of 3H-nucleocapsid RNA in the LNDV system revealed predominance of low molecular weight RNAs. Virus-specific sequences were shown to be present in fraction of DNA extracted by Hirt's method. Only the "supernatant" Hirt's fraction was infectious. The possibility of the presence of virus-specific sequences in free unintegrated form is discussed.