The expression of the junB gene parallels the expression of the MHC class I genes in Adenovirus (Ad) transformed cells. In Ad12E1-transformed primary BRK cells both genes are transcriptionally repressed only when the 13S product of Ad12E1A is present. This indicates that repression of MHC class I and junB genes is a function of conserved region 3 (CR3) of the Ad12E1A protein. In Ad5-transformed BRK cells expression of these genes is unchanged. In established NRK cells, however, introduction of Ad12E1A does not cause repression of the MHC class I and junB genes, but in these cells Ad5E1A increases the expression of both MHC class I and junB. Using mutant Ad5E1A genes, it is shown that this activation is mediated by CR1. Introduction of a functional junB gene under the control of a heterologous promoter in Ad12E1-transformed BRK cells causes no increase in MHC class I expression. This demonstrates that the down-regulation of junB is not directly responsible for class I repression, but rather that both genes are coregulated by the Ad12E1 region.