BHK 21 carrier cells persistently infected with VSV Indiana for over 2 years have been shedding generally very low levels of mature infectious virus or mature T particles (averaging less than one-hundredth p.f.u./cell/day) yet most cells are producing virus antigens and are resistant to homologous superinfection. However, large amounts of biologically active T particle RNP can be recovered from cytoplasmic extracts of these carrier cells even at times when they are shedding no detectable infectious virus. This recovered cytoplasmic RNP replicates (with helper B virions) to produce mature T particles, interferes strongly after DEAE dextran-facilitated uptake and, together with B virions, allows the establishment of a persistent carrier state in exposed cells. No 'provirus' DNA copies of the VSV RNA genome are detectable (less than 1/40 copy/cell or I copy per 40 cells) in carrier cells after more than 2 years of persistent infection, and all transfection attempts have failed using DNA from these VSV carriers or DNA from carrier cells persistently infected with some other negative strand RNA viruses (measles, mumps, LCM, influenza, rabies). Infectious viruses shed after more than I year from carrier cells originally infected with wild-type B virions are small plaque mutants showing a slight temperature sensitivity. Cured cell populations can be obtained from the long term VSV carrier culture by cloning in the presence or absence of antiviral antibody.