Persistent infection (PI) of YAC-1 cells by coxsackievirus B3 (CBV-3) was characterized. CBV-3 PIs were maintained for 7 months or more, although in two other cases cells were cured of virus at 6 and 6.5 months of PI. The titre of infectious virus peaked during the first week of the infection and then gradually decreased. The proportion of cells producing infectious centres increased to 100% by 48 h after infection, remained at that level up to the seventh day, and then rapidly decreased. Susceptibility to PI by CBV-3 varied widely among 40 clones from uninfected YAC-1 cells as judged by the yield of infectious virus at 6 weeks post-infection. None of the clones was completely lysed by the virus. Clones were not obtained from cells infected for 2 or 7 days. Of six clones obtained from cells infected for 14 days and 24 clones from cells infected for 6 weeks, none was producing virus and all were resistant to reinfection by CBV-3. Six of the clones were serially subcultured and all remained resistant for as long as they were maintained (5 months). During the course of the PI, viral variants which produced smaller plaques and required a longer incubation period for the development of visible plaques replaced the original viral population. Thus the PI involved a carrier culture with a large proportion of resistant cells. The resistant state did not require the continued presence of virus.