Abstract A culture of HeLa cells persistently infected with hemadsorption type 2 virus, designated as HeLa/HA2, was studied for its infectivity expression and cloning efficiency. The infectivity titer of cell suspensions approximates the number of carrier cells and is also approximately equal to the number of cells able to form clones. Very few virions are released into the medium. Both host and viral genomes can thus express themselves within each carrier cell. Studies to determine the mechanism by which virus passed from carrier to indicator cells revealed that the continued presence of antiviral serum prevented transmission. In contrast, treatment of carrier cells with the viral specific serum for an interval of less than 2 days followed by washing, did not inhibit transmission of the virus to monkey kidney cells. Therefore, successful transmission of infectivity from these carrier cells must frequently require more than 2 days of incubation.