In this study, SV5 infection of Balb/c mouse fibroblast cells has been used as a model system to investigate the possible mechanisms underlying the establishment and maintenance of Paramyxovirus persistence. It was found that following entry to these cells, the virus initiated a wave of transcription and replication, similar to that of a permissive infection, in which normal levels of each of the virus proteins were synthesized. However, by 48-72 hours post infection (p.i.) there was an almost complete cessation of virus mRNA and protein synthesis. Despite the decrease in virus activity, full length viral genome RNA and P and NP, the proteins involved in transcription and replication, could be detected at consistently high levels up to 5 days p.i., although the levels of HN, M, F and V declined. Immunofluorescence analysis supported these data showing that at later times p.i. although there were some cells positive for all the viral proteins, a high proportion of cells were strongly positive for NP, L and P, but negative for M, F and HN. In these cells, NP, L and P were often located in discrete cytoplasmic foci. These results suggested that the persistently infected cell population consisted of some cells in which the virus was active and other in which it was quiescent within cytoplasmic inclusions. A series of cell lines was established from a monolayer of Balb/c cells that had been infected at a high multiplicity. Immunofluorescence studies showed only a minority of cells in these clones to be infected with virus, indicating that during division, not all daughter cells became infected. Of the infected cells, some were positive for all the viral proteins, while others were positive for only NP and P. Co-cultivation of the cloned cells with Vero cells, which are permissive for SV5 replication, rapidly yielded non-defective virus, suggesting that the virus was active in some cells. These results suggested that the persisting virus was in a state of flux, able to reside as inclusions of inactive nucleocapids from which it could reactivate to initiate a new round of infection. Experiments aiming to determine if the persistently infected cells were resistant to immune attack demonstrated that cells at 5 days p.i., in which the majority of cells were quiescently infected, were less susceptible to immune lysis than cells at 1 day p.i. in which there was ongoing protein synthesis. Further experiments were carried out both to try to determine what had induced the persistent state in mouse cells and also to examine factors which might induce a similar state in different cell lines.