Nuclear chromatin was separated into four fractions according to solubility in low (0.2 mM MgCl2, 10 mM Tris-HCl, pH 7.6) and high (2 M NaCl) ionic strengths after digestion of nuclear DNA with nucleases. In nuclear matrix DNA the ratio of active to inactive genes was always higher than that in the original total DNA, i.e., 25 times greater in rat liver nuclei. In DNA released from the nuclei into a low ionic strength buffer the active to inactive gene ratio was lower than in total DNA (3.7 times as low in case of rat liver nuclei). The amount of carcinogens in matrix DNA exceeded that of DNA soluble in a low ionic strength buffer (3-4 times in case of rat liver nuclei and 16 times in case of hamster embryo cells). The two other fractions occupied an intermediate position between the above said fractions of DNA. The experimental results suggest that the level of carcinogen-induced modifications may be increased in active genes, including transcribed protooncogenes.