The luciferase reporter activity in VSV*DG(Luc)-infected cells depends on the replication/transcription levels of the RNA replicon. Thus, a reduction in viral genome replication as a consequence of type IFN action should lead to correspondingly lower luciferase levels. To test this hypothesis, normal human dermal fibroblasts (NHDF) were incubated for various time periods with serial dilutions of human IFN-b before infection with VSV*DG(Luc). A dose-dependent effect of IFN-b on firefly luciferase reporter activity was observed after 1 hour of incubation (Fig. 2a) . About 1 unit of IFN-b led to 50% suppression of luciferase activity. Extending the incubation time to 2 hours did not further improve the sensitivity of the test (p.0.05 for the 0.08 to 1.25 IFN units range). However, if the cells were treated for 20 hours, the dose response curve was significantly shifted to lower IFN units (p#0.001 for the 0.02 to 20 units range). Approximately 0.05 units of IFN-b were now sufficient to suppress reporter activity by 50%. These results indicate that an IFNb-induced antiviral state in NHDF cells can be detected as early as 1 hour after addition of IFN-b, although the sensitivity of the assay is higher if the cells were incubated with IFN-b for prolonged time.