Mouse cells with an established human cytomegalovirus (HCMV) infection were fused with susceptible human embryonic fibroblast cells. CMV-specific early antigens could be demonstrated in the cytoplasm and cell-membrane of the heterokaryons. Treatment with 5-iodo-2'-deoxyuridine (IUdR) of the heterokaryons or of the latently infected mouse cells, prior to their fusion with human cells, could induce the appearance of immunoflourescent elements, characterised as late antigens, and of infectious virus. Our data show that the mouse cells, in the latent stage of infection, contain the whole virus genome and that the replication of the virus is controlled by a genetic mechanism of the host cells both in virus-harbouring mouse cells and in heterokaryons.