Interferons (IFN) have cancer suppressor activities for many transformed cells; however, IFN does not suppress transformation by SV40 large T antigen. The studies described in this paper therefore evaluated the effect of IFN on SV40-promoted gene expression in 3T3T cells. The results show that SV40-promoted gene expression can be induced 200% or three-fold by Type I IFN treatment regardless of whether beta-galactosidase or chloramphenicol acetyltransferase (CAT) is used as the reporter gene. This IFN effect is dosage-dependent, requires 24-48 h of exposure for maximum induction of CAT activity, and is probably not due to a post-transcriptional effect of IFN on CAT because IFN has no effect on CAT expression driven by thymidine kinase promoter. The induction of SV40 early transcription by IFN does, however, require that the integration of plasmid within the cell's genome. Additional data specifically show that the SV40 promoter is required for IFN's effect because IFN will not induce CAT if the SV40 enhancer is inserted upstream of thymidine kinase promoter to control expression of CAT gene.