Abstract: The present invention is directed to immortalized STAT1-deficient mammalian cell lines. STAT1 is a signal transducer and activator of transcription that becomes phosphorylated when cells are treated with type I or type II interferons and leads to induction of specific gene expression, resulting in establishment of the antiviral state and the other known biological responses to interferons, including the inhibition of cell proliferation. Cells which lack this gene product are useful for producing high titers of viral stocks, for producing recombinant viral vectors, for testing samples, especially clinical samples for the presence of virus and for screening candidate compounds or drugs for anti-viral activity.