Interferons (IFNs) are potent biologically active proteins synthesized and secreted by somatic cells of all mammalian species. They have been well characterized, with respect to structure, biological activities and clinical therapeutic effects. Historically, IFN antiviral activity assays were the first type of bioassays developed to measure the relative activity or potency of IFN preparations. Here we report a new virus free, cell-based assay to quantify murine type I IFN. It basically consists of an indicator cell line in which the Cre-recombinase is driven by the IFN- inducible Mx1 promoter and, when activated, deletes a stop cassette upstream of the eGFP coding region, resulting in the expression of eGFP. The percentage of eGFP expressing cells accurately correlates to the amount of type I IFN added to the culture and can easily be monitored.