The use of external guide sequence (EGS) in directing endogenous ribonuclease P (RNase P) for inhibition of viral propagation is described in this chapter, with an emphasis on chemically modified EGSs and their extracellular delivery. Targeting of the mRNA-encoding human cytomegalovirus (HCMV) protease by DNA-based EGSs is presented as an example of how to design chemically modified EGSs for antiviral applications. General information about the EGS-based technology is included, followed by detailed protocols for EGS design, human RNase P purification, in vitro assay of EGS activity, liposome-mediated delivery of chemically modified EGSs and detection of their distribution in cells, and an assay of EGS activity for blocking growth of HCMV in cultured cells.