Treatment of NIH 3T3 fibroblasts with interferon-alpha or interferon-gamma (IFN-alpha or IFN-gamma) significantly reduced murine cytomegalovirus (MCMV) replication. Determination of viral DNA in the nuclei of the infected cells before onset of DNA replication demonstrated that virus uptake, transport to the nucleus, and DNA stability were not decreased. Analysis of the virus specified mRNAs soon after infection revealed that in the cells exposed to IFNs expression of the immediate early (IE) genes was strongly reduced. Nuclear run-off transcription analysis showed that this inhibition is due to significant reduction of IE gene transcription rates following IFN treatment. Since transcription of the MCMV IE region is regulated by a strong enhancer element, a construct containing the chloramphenicol acetyltransferase (CAT) reporter gene, driven by an 1.2 kb segment spanning the enhancer and IE1/3 promoter region of the IE transcription unit, was transfected into NIH 3T3 cells. Treatment with IFN-alpha or IFN-gamma after transfection strongly reduced CAT activity compared to untreated controls. In an attempt to define a negative IFN-responsive element in the IE enhancer, a series of deletion mutants driving the CAT reporter gene were transfected into NIH 3T3 cells that were then treated with IFN-alpha. With the sole exception of the construct containing the minimal MCMV IE1/3 promoter (-102 to the cap site), all other deletion mutants were strongly down-regulated by IFN-alpha-treatment. Taken as a whole, these results suggest that IFNs inhibit MCMV replication by impairing the transcription of the IE transcription units, and that this negative regulation is carried out by sequences scattered throughout the IE enhancer region.