SUMMARY Infectious virus could not be recovered by disruption of human embryonic fibroblast cultures inoculated with cytomegalovirus and treated with cytosine- arabinoside (ara-C), either during treatment or 4 to 5 days after removing ara-C. However, when intact cells were used for infection, at least I in 7o ceils was found harbouring virus. Examination by immunofluorescence revealed that 4 ° to 5 ° % of the cells contained virus-specific antigens in the form of small granules diffusely distributed in the nuclei. The presence of infectious virus could be detected even in disrupted cells 4 to 5 days after removing ara-C; intracellular antigens of every kind were also found to develop. Experiments showed that the cultures continued to harbour latent infection indefinitely in the presence of ara-C. Cultures harbouring latent virus were susceptible to superinfection with cytomegalovirus.