The author reports the mean DNA values of interphase nuclei in 25 human tumours of epithelial origin, not previously treated. These values are closely related to the chromosome numbers. The constancy of the observed values for each tumour makes it likely that each tumour represents a clone, derived perhaps from one single cell, in which the genetic changes first occurred. The analysis of tumour cell karyotypes indicates, in agreement with DNA estimations, that the mean chromosome size in tumour cells is larger than normal. The data confirm the view that most of the variations present in tumour cells are not only structural but also represent an increase of the active DNA content per chromosome. The practical implications of these findings are pointed out.