UltraPure DNA solutions from Calf Thymus, Herring Sperm, or Salmon Sperm are ready-to-use, sheared DNA solutions that are used directly in the preparation of prehybridization and hybridization solutions. These prepared solutions are used to block the non-specific attachment of probe to the surface of a membrane. The DNA solutions are prepared from highly pure, phenol/chloroform-extracted DNA and DNase-free and RNase-free (DEPC-treated) distilled, deionized water and are sheared to an average size of ≤2,000 bp. The concentration is adjusted to 10 mg/mL.