Cytopathic changes and virus-specific antigens developed in, then disappeared from, mouse fibroblasts infected by a strain of human cytomegalovirus (CMV), but their disappearance was delayed in cells treated with idoxuridine prior to infection. The replication of vesicular stomatitis virus and herpes simplex virus was restricted in human CMV-infected mouse cells as long as human CMV-specific antigens were present. Virus-specific antigens could be induced by treatment with idoxuridine or arginine deficiency in mouse cells which had previously turned "negative".