Vesicular stomatitis virus (VSV) mutant T1026R1 of the Indiana (IN) serotype is a good inducer of interferon (IFN). This mutant was used to study the activation of NF-kappaB, a transcription factor necessary for IFN induction, in mouse L929 cells that were stably transfected with a chimeric gene containing the human IFN-beta gene promoter attached to the chloramphenicol acetyltransferase (CAT) coding sequence. NF-kappaB DNA binding activity was detected as early as 30 min after virus adsorption in nuclear extracts, increased up to 4 hr, and then remained constant for at least 6 additional hr. The kinetics of CAT expression correlated with the kinetics of NF-kappaB nuclear DNA binding activity. Virus entry and delivery of viral components into the cytoplasm were required for NF-kappaB activation. Exposure of T1026R1 to one hit of UV irradiation nearly completely reduced NF-kappaB activation. In cells infected with wild-type (wt) VSV (IN), a noninducer of IFN, NF-kappaB DNA binding activity in the nucleus was delayed for several hours after virus adsorption. Coinfection of wt VSV and T1026R1 resulted in the reduction of T1026R1-promoted NF-kappaB activation. This inhibitory activity of wt VSV was abolished by one hit of UV irradiation. Under similar conditions expression of the CAT gene was more UV resistant, suggesting that IFN gene expression is regulated at multiple levels.