HeLa cells infected with types 1 or 4 adenovirus were separated into cytoplasmic and nuclear fractions by mechanical disruption and differential centrifugation and the quantity of infectious virus in each was determined. The results showed that the majority of infectious virus of both types could be isolated in the cytoplasmic fraction. It was not possible to explain the large amount of type 4 virus in the cytoplasmic fraction by the number of nuclei disrupted in the fractionation procedure, but the amount of type 1 virus in the cytoplasmic fraction could have been contributed by disrupted nuclei. This suggested that there might be a basic difference in the intracellular formation of the two types of virus. The intracellular distribution of complement-fixing antigen was similar to that of infectious virus in type 4-infected cells. Technical difficulties, inherent in cellular fractionation studies, were encountered but did not appear to explain the results obtained.

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