Virus and cell DNA were separated by isopycnic centrifugation after subjecting a mixture to denaturation and reannealing. Virus DNA renatured, while cell DNA retained the density of single-stranded material. This process magnified the originally very small density difference between the two DNA's. The method's usefulness was demonstrated with mixtures of pure virus DNA and pure cell DNA, and was applied to the study of DNA synthesis in vaccinia infected cells.