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Sendai virus from L cell harvests was adsorbed to chicken erythrocytes or L cells and fixed smears were stained by the complement immunofluorescent technique. The numbers of the fluorescent particles corresponded linearly to the virus hemagglutinin units adsorbed from suspensions. Excellent correlation was found between the numbers of fluorescent units and the counts of particles adsorbed to chicken red cells as examined by electron microscopy.

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.

A majority of chick embryo cells have previously been shown to contain a genetic element called chick-cell associated helper factor that is responsible for the presence of the helper function for the formation of an infectious form of the Bryan strain of Rous sarcoma virus. The helper factor was unable to mature into complete virus by itself, but, with the assistance of other leukosis viruses, its genome was recovered as a new Rous-associated virus-60. The present study demonstrated a good correlation between high levels of helper function and production of a group specific antigen to avian tumor viruses. Among the minority of embryos lacking detectable amounts of the antigen, about 40% possessed the helper function at low levels. By the use of a sensitive method, Rous associated virus-60 could also be recovered from chick embryo cells that are negative for the group-specific antigen. It is suggested that all chick embryos contain the genetic information for the helper factor; its expression is controlled by a regulatory gene.

A culture of HeLa cells persistently infected with hemadsorption type 2 virus (HeLa/HA2) was shown to have a similar plating efficiency to that of uninfected HeLa cells, demonstrating the infection was not pathologic. Each of the cells from derived clones contained virus antigen when examined by immunofluorescent staining. When mixtures of HeLa/HA2 and normal HeLa cells were seeded in a single dish in the absence of antiviral serum, and allowed to form clones, the immunofluorescent positive and negative clones were easily differentiated. The ratio of the antigen bearing to nonbearing clones was very close to that of the number of HeLa/HA2 and normal HeLa cells in the original mixture, indicating that the virus antigen of HeLa/HA2 cells could not transfer to the normal HeLa cells. When the HeLa/HA2 clones were grown in medium containing specific antiviral serum, the serum did not affect the formation of the virus antigen in the cell. Therefore, the persistent infection was maintained by transfer to daughter cells during cell division.