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Cloned 3T3FL cells were synchronized in G1 phase of the cell cycle by deprivation of multiplication stimulatory activity of serum and were then infected with Moloney leukemia virus. Eclipse period of virus could be made to vary from less than 10 to 34 h. All virus release was completely dependent and occurred immediately after the first mitosis following serum reconstitution. Virus yield was not affected by the time of virus inoculation as related to the cell DNA synthetic phase. Colchicine arrested the cells in mitosis and prevented the formation of infectious virus. Viral proteins p10, p30, and gp71 were assayed in cell lysates during the growth curve of virus in synchronized cells. The group-specific determinants of each protein were measured in a competition radioimmunoassay. None of the virus proteins appeared during the eclipse period of the virus. All three proteins appeared simultaneously, coincident with mitosis, and continued to rise during the G1 phase. The absolute quantities of each protein were proportional to the amount of Moloney leukemia virus produced. The relative amounts of some of the viral proteins in the cell did not correspond to their content in purified virions suggesting several possible mechanisms of control.

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