This product is a convenient, ready-to-use solution of calf thymus DNA especially prepared for use in the preparation of pre-hybridization and hybridization solutions and as a DNA carrier in yeast transformation protocols and other related methods. This product is also an excellent substrate for deoxyribonuclease or DNA polymerase. This solution contains sheared single stranded DNA molecules that can be used to block the non-specific attachment of probe DNA to the surface of a membrane (Southern) or to increase yeast transformation efficiency. Prepared by a modification of the method of Emanuel and Chaikoff, JBC 203, 164 (1953) from calf thymus DNA by mechanical shearing and heat denaturation to an average size of 100 to 2000 base pairs. To reverse any renaturation occurring during storage this material should be briefly boiled and rapidly chilled prior to use.