This protocol describes the preparation of and treatment with DNase I to degrade DNA in solutions containing iron chloride, EDTA–Mg ascorbate buffer, and cesium chloride. DNase I treatment, CsCl purification, and sucrose purification methods were compared using replicated viral metagenomics in Hurwitz et al. 2012. We use this protocol primarily to remove free host and other contaminating DNA from phage preparations prior to extracting nucleic acid from the phage particles.