Complementary DNA, synthesized with L-cell polyadenylated RNA as template, renatured with total L-cell DNA to about 70%. About 30% complementary to unique sequence DNA and another 10 and 30% corresponded to sequences about 20- and 500-fold repetitive. Complementary DNA was fractionated after partial hybridization with total polyadenylated RNA to obtain preparations enriched or impoverished in complements of the most frequent polyadenylated RNA. Renaturation of these complementary DNA fractions with L-cell DNA revealed that most frequent RNAs are transcribed from repetitive DNA sequences, Complementary DNA, density labeled with bromodeoxyuridine, was fractionated by renaturation with L-cell DNA to yield fractions enriched in repetitive and unique sequence DNA. The denisty labeled complementary DNA was purified by equilibrium centrifiguation in an alkaline Cs2SO4 gradient. The complementary DNA representing mainly repetitive DNA sequences hybridized preferentially to frequent polyadenylated RNA.