Murine cytomegalovirus downregulates expression of MHC class I molecules required for recognition of virus-specific CD8+ CTL, effector cells which mediate clearance of virus from the host. We previously identified two mechanisms of MHC class I downregulation in MCMV-infected immortalized fibroblasts: a defect in transport of the class I molecules from the endoplasmic reticulum to the cell surface, and a significant inhibition in class I heavy chain synthesis. We now report that these two mechanisms are independently regulated at early times post MCMV infection. The defect in MHC class I transport, evident at 4 hr postinfection, precedes the defect in synthesis at 6-8 hr postinfection. Levels of MHC class I heavy chain mRNA decline between 4-12 hr postinfection. Levels of mRNA for some cellular genes also declined at approximately the same rate in MCMV-infected but not mock-infected cells. The defect in MHC class I synthesis and transport was evident in several fibroblast cell lines immortalized by a variety of agents. However, only the defect in transport of MHC class I heavy chain was seen in primary fibroblast cells. In primary cells, synthesis of heavy chain molecules was unaffected by MCMV infection. Thus, MCMV infection alters expression of host cell proteins at multiple levels in a cell-specific manner, perhaps dependent upon the state of differentiation or transcriptional activity of the cell.