Most preparations of infectious virus nucleic acids retain only about 0.1% or less of the infectivity of the original virus material. This is not necessarily the result of destruction of much of the intact nucleic acid during chemical procedures, but may be due to the insensitivity of the assay methods employed (Holland 1961; Tovell & Colter, 1967). For the titration of infectious DNA obtained from polyoma virus, one of the methods in current use requires the direct plating of DNA in 0.55 -NaCl on the monolayers of mouse embryo cells (Weil, 1961). This procedure has the disadvantage that the cell damage caused by the hypertonic NaCl renders accurate plaque counts difficult. A more recently described method (Warren & Thorne, 1968) avoids this complication by infecting monolayers in the presence of diethylaminoethyldextran (DEAE-dextran) in an isotonic medium.


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