Abstract Although relatively resistant to infection with Coxsackie A9 virus of monkey kidney (MK) culture origin, HeLa cells have been found capable of supporting growth of this virus. When heavy inocula of the A9 virus were used in cultures of HeLa cells adapted to growth in medium containing 10% human, horse, or calf serum, the cultures showed viral cytopathogenic effects (CPE) but recovered after addition of fresh growth medium. These cultures could be passed serially for many cell generations with consistent recovery of the virus in the supernate. The term “carrier culture” has been used to designate this type of virus-cell relationship ( Lwoff, 1953 ). No evidence for a lysogenic state was found in this system. Animal or human immune serum eliminated virus from the cultures and clonal isolates of cells from the carrier culture were negative for virus. Only a small proportion of the cells in carrier cultures were infected. Prolonged cultivation of the carrier cultures tended to select not only cells with increased resistance to Coxsackie A9 infection, but also virus with altered properties. Virus from carrier cultures differed from the original virus grown in MK by its (a) lack of virulence for suckling mice, (b) increased virulence for HeLa cells, (c) slow plaque formation in MK plaque plates, and (d) antigenic composition. The evidence presented indicates that the principal factors responsible for continued maintenance of the carrier cultures are the relative insusceptibility of HeLa cells to Coxsackie A9 virus, especially in the state of cellular activity present in growth medium, and the prevention of high multiplicities of virus in the cultures by rapid thermal inactivation.