The E1 region of human adenovirus type 12 (Ad12 E1) represses the expression of major histocompatibility complex (MHC) class I genes in transformed primary rodent cells. Conflicting results have been reported as to whether this E1A-mediated repression occurs at a transcriptional or a posttranscriptional level. In the present study, we show that in Ad12 E1-transformed primary baby mouse kidney cells from transgenic mice harboring the human growth hormone gene under control of an H-2K promoter both the expression of the endogenous MHC class I genes and the expression of the transgene are repressed. This experiment, as well as nuclear run-on analyses performed with single-stranded probes, revealed that Ad12 E1A inhibits MHC class I gene expression by repressing its promoter.