The effects of a hundred-fold difference in virus/cell multiplicity on the interaction of Newcastle disease virus (NDV) with HeLa cells were studied, and various phases of the virus reproductive cycle were related to cellular consequences of infection. At both multiplicities used all cells were infected. The following events occurred 1 to 2 hours earlier in cells which were inoculated with the higher multiplicity: (a) first appearance of newly made virus antigen, and the amount present at any time during the period of rapid increase; (b) onset and time course of production of infective virus; (c) development by infected cells of hemadsorbing ability; (d) onset and time course of inhibition of mitosis; and (e) onset and time course of marked cell damage.

Double infection of HeLa cells with NDV and NWS was demonstrated by the fluorescent antibody technique, and was used to show that the establishment of interference against NWS was also dependent upon the multiplicity of NDV.

In cells inoculated at each multiplicity, newly made virus antigen appeared at the same time as the first infective virus particles. Infective virus rapidly reached a peak, and then declined. Viral antigen continued to increase for several hours after the decline in infective virus had begun. Thus, only a small fraction of the virus antigen produced was incorporated into new infective particles. The maximal yield of such particles was only 6 to 11 per HeLa cell. Over 95 per cent of new virus was cell-associated, but could be neutralized by treatment with antiserum before disruption of cells.

Mitosis occurred in cells which had produced and released infective NDV. Progressive inhibition of mitotic activity in infected cells was correlated with continued production of viral antigen. Marked cytopathic changes developed after mitotic activity had decreased to low levels.

The mechanism by which NDV inhibits mitosis in HeLa cells is discussed.

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