The mechanism of assembly of Newcastle disease virus in chick embryo cells was investigated in two series of experiments. When protein synthesis was inhibited by addition of puromycin, cycloheximide or fluorophenylalanine at any time during the course of infection, subsequent virus production was soon inhibited. These drugs were inhibitory even when they were added to the cultures as late as 10 hr after infection, when large amounts of virus precursor proteins were present within the cell. In the second series of experiments, the kinetics and efficiency of incorporation of radioactive amino acid into virus particles were examined by the pulse-labelling technique. A 30 min. labelling period at the 3rd or 6th hr of infection resulted in the release of highly radioactive virus during the period of 1½ hr immediately after the pulse. However, when pulse-labelling was performed at the 9th hr, the maximally labelled virus was found in the yield obtained 3 hr after the pulse, and the specific radioactivity of virus was less than 1% of that of the virus harvested from cultures labelled earlier. On the basis of these findings, possible mechanics of virus assembly are discussed.


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