Monocyte/macrophage cells can be infected with human cytomegalovirus (HCMV) and could be a reservoir and a vehicle for virus spread in infected patients. The monocytic-like cell line THP-1 differentiated along the monocyte pathway with a phorbol ester (phorbol 12-myristate 13-acetate, PMA) for 24h was infected with cell free-supernatant of Towne CMV-strain infected fibroblasts MRC-5. We designed experiments to study the replication of HCMV in PMA treated THP-1 cells, exposed to DHPG (Cymevan, ganciclovir, Syntex). We incubated PMA treated THP-1 cells with DHPG for one to five days at 37 degrees C before or after the infection. The cells were cultivated for 5 days at 37 degrees C then supernatants of culture were collected and they were added to fibroblast cultures. The infection of the fibroblasts was studied on the basis of immune detection of viral immediate early antigens (IEA) in nucleus 24h after culture. Incubation of infected THP-1 cells with 11.1 microM DHPG for 5 days at 37 degrees C resulted in 95 +/- 2% inhibition of the replication whereas incubation of the infected cells with the same concentration of DHPG for two days resulted in 50 +/- 5% inhibition. Pretreatment of cells with DHPG for 5 days induced a 50 +/- 4% decrease of the level of infectious particles produced by HCMV infected cells. This is the first report of the antiviral effect of DHPG in macrophagic-like cells.