Following infection of BALB/c fibroblastic (BF) cells with simian virus 5 (SV5) only low levels of infectious virus were produced and the majority of cells survived the infection. However at 1 day post-infection (p i ), near normal levels of all the virus proteins were synthesized and the virus genome was replicated. RNA analysis of the infected cells revealed that the levels of viral genomic RNA remained high over 5 days of infection, but that viral mRNA levels were significantly reduced by 3 days p.i. There was no evidence for the accumulation of defective genomes over this period. The reduction in mRNA levels was reflected by a concomitant decrease in the rate of ongoing viral protein synthesis. Despite the apparent decrease in viral transcription, comparative measurements of the relative levels of the different virus proteins at various times p.i. revealed that the levels of the P and NP proteins were similar at 1 and 5 days p.i. but the levels of Y, M and F declined. Immunofluorescence analysis supported this data showing that at later times p.i., although there were some cells which were positive for all the viral proteins, a high proportion of cells were strongly positive for NP and P but negative for M, F and HN proteins. In these cells, NP and P were often located in discrete cytoplasmic foci. A series of cell lines were established from BF cells that had been infected at high multiplicity. Immunofluorescence studies showed that only a minority of cells in these cell lines were infected. This suggests that upon cell division, in a proportion of cells, virus replication was not taking place; otherwise it would be expected that all the daughter cells would remain infected. However, upon co-cultivation of these cells with Vero cells (cells that are fully permissive for SV5 replication), non-defective virus could be recovered. Virus cytoplasmic inclusion bodies could still be detected in a small proportion of BF cells that had been infected at high m.o.i. and passaged 10 times over a 12 week period, and again low levels of infectious virus could be recovered from these cells. It is proposed that in these persistently infected cells, the majority of virus genomes reside in an inactive form in cytoplasmic inclusion bodies but from which virus may occasionally be reactivated. Studies on cell clones obtained from a Vero cell line persistently infected with SY5 for over 100 passages (in which there are large numbers of defective genomes) also showed that not all daughter cells remained infected upon cell division. At later times p.i. in BF cells, when the majority of cells were positive for only NP and P, the cells became more resistant to cell-mediated immune lysis. The significance of these results in terms of the biology and immunology of paramyxovirus infections is discussed.