Lymphocytic choriomeningitis virus (LCMV) infection of most tissue culture cell lines results in a non-cytopathic persistent infection. Persistent infections in vitro share many characteristics with persistent LCMV infection of mice; both are associated with decreased titres of infectious virus, restricted accumulation of viral glycoproteins at the surface of infected cells and the generation of interfering particles. We have used gel electrophoresis and hybridization techniques to analyse LCMV gene expression during persistent infection of a number of tissue culture cell lines. Our study has demonstrated that, although deleted viral RNAs can be detected during persistent LCMV infection in vitro, there may not be an obligatory association between deleted RNAs and persistence. In addition, we have found that LCMV interfering activity can be produced in the apparent absence of deleted intracellular viral RNAs.