DNA was prepared from 15 different mouse and rat cell lines transformed by chemical carcinogens in vitro and in vivo. These DNAs were applied to NIH3T3 mouse fibroblast cultures by using the calcium phosphate transfection technique. DNAs of five donor lines were able to induce foci on the recipient monolayers. Ten other donor DNAs yielded few or no foci. DNAs from control, nontransformed parental cell lines induced few or no foci. Chromosomes were transfected from one donor whose naked DNA was unable to induce foci, and morphologic transformation of recipients was observed. These experiments prove that in five of these cell lines the chemically induced phenotype is encoded in DNA, and the sequences specifying the transformed phenotype behave as a dominant allele in the NIH3T3 recipient cells. The sequences encoding the transformation are likely found on a single fragment of DNA.