After an initial acute infection with cell killing, chicken or duck embryo fibroblasts infected in culture with reticuloendotheliosis viruses set up a chronic infection with no cell killing or morphological transformation. Essentially all of the chronically infected cells produced virus. The virus production was not sensitive to cytosine arabinoside or mitomycin C as was virus production in an acute infection. The chronically infected cells had a strong group-specific resistancto the c.p.e. of superinfecting reticuloendotheliosis viruses. However, they were sensitive to vesicular stomatitis virus and avian leukosis-sarcoma viruses. After double infection, single cells produced reticuloendotheliosis virus and avian leukosis-sarcoma virus.