The cells from fetuses infected in utero with rubella virus were analyzed for the presence or absence of virus. Cells that were virus-free were examined for evidence of prior experience with virus. By infectious center assay of fetal cells in GMK tubes, and by growing fetal cells in culture tubes, it was established that not more than 0.1% of the fetal cells contained infectious virus. No evidence of experience with rubella virus was found in virus-free cells, for such cells were able to support the replication of rubella virus, Newcastle disease virus, and vesicular stomatitis virus. The production of interferon and sensitivity to interferon was normal in virus-free cells, but not in virus-containing cells. The data obtained demonstrate that congenital rubella differs from congenital lymphocytic choriomeningitis virus infection of mice where all of the cells contain viral antigen or are resistant to superinfection by the virus.