Persistent infections with several strains of mumps virus (strains Torii and Miyahara), measles virus (strains Edmonston, CAM-70, AIK-C and Schwarz) and subacute sclerosing panencephalitis (SSPE) virus (Hälle and Mantooth) were established in various cell lines (FL, KB, A549, SK-AS, 293, K562, Ramos and NC-37). Oligo-2',5'-adenylate synthetase activity was demonstrated to be only slightly induced by interferon in cytoplasmic and nuclear fractions of cell lines persistently infected with mumps virus. In these cells, resistance to vesicular stomatitis virus infection was not induced by interferon treatment. Treatment of the persistently infected cells with interferon for 10 and 24 h did not stimulate an increase in the amount of synthetase mRNA. In cells persistently infected with measles and SSPE viruses, reduced induction of the enzyme varied with host cell types. Induction of the enzyme was not found in K562, SK-AS and KB cells, but was recognized in NC-37 and FL cells.