Deoxyribonucleic acid (DNA) synthesis and virus growth in frog virus 3 (FV-3)infected mammalian cells in suspension were examined. The kinetics of thymidine incorporation into DNA was followed by fractionating infected cells. The cell fractionation procedure separated replicating viral DNA from matured virus. Incorporation of isotope into the nuclear fraction was depressed 2 to 3 hr postinfection; this inhibition did not require protein synthesis. About 3 to 4 hr postinfection, there was an

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... ease in thymidine incorporation into both nuclear and cytoplasmic fractions. The nuclear-associating DNA had a guanine plus cytosine (GC) content of 52%c; unlike host DNA it was synthesized in the presence of mitomycin