We studied the effect of a novel purine acyclic nucleoside, 9-(1,3-dihydroxy-2-propoxymethyl)guanine (DHPG), on human cytomegalovirus (HCMV) replication. The susceptibility of HCMV to this drug was monitored in cell culture by plaque reduction assay. HCMV replication of various strains was inhibited to the extent of 50% by 1 to 5 microM DHPG. DHPG was highly specific in its anti-HCMV activity, since at concentrations as high as 100 microM it did not exert any detectable inhibitory effect on uninfected cell macromolecular synthesis and cell growth. At concentrations of 2 to 4 microM, the drug inhibited the synthesis of six virus-specific polypeptides with molecular weights of 200,000 (VP200), 150,000 (VP150), 67,000 (VP67), 54,000 (VP54), 32,000 (VP32), and 27,000 (VP27) up to 96 h after infection. HCMV DNA synthesis was also considerably suppressed at concentrations of 2 to 4 microM DHPG. Upon removal of the inhibitor, however, viral DNA synthesis resumed and infectious virus reappeared, indicating that this inhibition was a virostatic reversible-type inhibition.