Human cytomegalovirus (HCMV), a betaherpesvirus, is a pathogen which escapes immune recognition through various mechanisms. In this paper, we show that HCMV down regulates gamma interferon (IFN-γ)-induced HLA-DR expression in U373 MG astrocytoma cells due to a defect downstream of STAT1 phosphorylation and nuclear translocation. Repression of class II transactivator (CIITA) mRNA expression is detected within the first hours of IFN-γ–HCMV coincubation and results in the absence of HLA-DR synthesis. This defect leads to the absence of presentation of the major immediate-early protein IE1 to specific CD4+ T-cell clones when U373 MG cells, used as antigen-presenting cells, are treated with IFN-γ plus HCMV. However, presentation of endogenously synthesized IE1 can be restored when U373 MG cells are transfected with CIITA prior to infection with HCMV. Altogether, the data indicate that the defect induced by HCMV resides in the activation of the IFN-γ-responsive promoter of CIITA. This is the first demonstration of a viral inhibition of CIITA expression.