Human cytomegalovirus (HCMV) represents one of the most medically important human viruses and causes a wide spectrum of human diseases, including birth defects and mental retardation in newborns, common opportunistic infections in acquired immunodeficiency syndrome (AIDS) patients (e.g., CMV-associated retinitis and pneumonia), and possibly cardiovascular diseases such as atherosclerosis. This chapter describes the utilization of RNase P ribozyme-specifically, M1GS ribozyme, as a gene-targeting agent for blocking HCMV gene expression and growth. The target for the RNase P ribozyme is the overlapping region of the mRNAs that code for HCMV major transcription factors IE1 and IE2, which are essential for viral gene expression and replication. The methods described in this chapter focus primarily on i) construction of the retroviral vector for expression of M1GS ribozymes in cultured cells, ii) generation of stable cell lines expressing ribozymes, iii) determination of the expression of M1GS RNAs in human cells, and iv) evaluation of the efficacy of ribozymes in inhibiting HCMV IE1/IE2 expression and viral growth. Using these methods, we successfully constructed M1GS RNAs against the IE1/IE2 mRNA sequence and recently showed that a reduction of up to 150- to 3000-fold in HCMV growth is found in cells that express the ribozymes.