Thein vivointeractions of trenbolone and zeranol with rat‐liver DNA were assayed by determining their covalent‐binding indices (CBI), which correlate well with the hepatocarcinogenic potencies of chemicals. As controls, 17ß‐oestradiol and testosterone were used. Sixteen hours after intraperitoneal injection of the [3H]‐labelled compounds (specific radioactivity ∼50 Ci/mmol) the animals were sacrificed and the DNA was isolated from the livers and deproteinized by several Sevag extractions and proteinase digestion. Contaminating RNA was eliminated by pancreatic RNAase digestion. Finally, the DNA was further purified by chromatography on hydroxylapatite and by centrifugation in a CsCl gradient. Decreased CBI values after each step of purification showed that extensive purification was essential.