The events during the release of measles virus from latently infected hamster cells after co-cultivation with BSC-1 cells were studied by immunofluorescence and electron microscopy techniques. Before co-cultivation, measles virus antigens (nucleocapsid) were distributed in an apparently random fashion throughout the cytoplasm. Six hours after co-cultivation with BSC-1 cells, the measles virus nucleocapsid became aggregated in close proximity to the nuclear membrane. These antigens then diffused towards the cell periphery, and progeny virus was observed budding from the cell surface by 16 h after co-cultivation. Fusion of the BSC-1 cells to the latently infected hamster cells was necessary for virus release to occur, and an intact, viable BSC-1 cell was also required. A possible mechanism for the block of virus replication in the latently infected cells is discussed.