Abstract
Preparations of polyoma virus purified by CsCl isodensity centrifugation stained yellow green at pH 3.8 with 0.01% acridine orange. Pretreatment with a proteolytic enzyme was necessary before development of the stain could be inhibited by DNAase. RNAase had no effect on similar preparations. These characteristics are consistent with the identification of the nucleic acid of the mature polyoma virus particle as a double-stranded DNA.