.5 mM of novobiocin completely suppresses the replication of human cytomegalovirus (HCMV) in vitro. Based on the evidence that murine cytomegalovirus (MCMV) titres were reduced in vitro by 60% with the same concentration of the drug, the in vivo antiviral activity of novobiocin has been tested in the murine model. The results showed that if treatment started before the infection virus was not isolated from parenchymal organs during treatment. Virus could be isolated 24 h after stopping treatment, but not from kidney or spleen.