The topological linking numbers in closed superhelical loops of nuclear DNA were measured as the density of DNA topological turns ('titratable superhelical turns') per unit length of DNA by means of sedimentation of superhelical DNA (in nucleoids) in gradients of 15-30% sucrose containing 1.95 M NaCl and various concentrations of ethidium bromide. In four malignantly transformed Syrian hamster cell lines (three SV40-transformed and one spontaneous) the density of DNA topological turns was equal to or higher than the density of DNA topological turns in early passage embryonal Syrian hamster cells (/delta/ greater than or equal to 0.076) and, in contrast to normal cells, the malignant ones did not decrease the density of their DNA topological turns upon cultivation. It is proposed that the persistence of high densities of DNA topological turns in malignant cells is responsible for activation of transcription of a number of genes during malignant transformation.