Virus yield/cell from Balb-c 3T3 monolayer cultures infected with polyoma virus was found to decrease with increasing cell density. Close cell contact did not inhibit yield nor did moderate depletion of medium constituents by cell growth. Adsorption of input virus/cell decreased as the cultures aged.

Addition of fresh medium to resting cultures provoked a delayed increase in susceptibility. Treatment of resting cultures with trypsin initially enhanced their susceptibility, but a gradual decline to the original level followed; adsorption of input virus followed a similar course. These and other results described are consistent with the interpretation that only cells in a state of active division are susceptible and that resting cells acquire a trypsin-sensitive surface component which blocks infection.

Yields of virus from sparse cultures of several clones tested varied. Study of the clone of highest susceptibility revealed that cell DNA synthesis was not induced by virus infection and that the total amount of virus DNA made was small and closely equivalent to the amount found encapsidated.


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