Conventional type IFN bioassays often take 24 hours or more to be completed as they rely on the inhibition of cytopathic effects. In contrast, the bioassay presented here takes advantage of the firefly luciferase reporter, which can be detected with high sensitivity long before a cytopathic effect is apparent. Although the luciferase reporter enabled us to unambiguously quantify the IFN-mediated reduction of virus replication, we frequently observed that the standard deviations for the antiviral activities increased when the cells were treated with low amounts of type I IFN. This likely reflects the high reporter gene expression at low levels of type I IFN and the inaccuracy associated with pipetting small volumes (6 ml).