The relationship between the modal DNA content of human tumour cells in interphase and the ploidy of the tumours has been discussed. Data on chromosome number, the DNA content of dividing cells and the number of sex chromatin bodies per cell have been considered in relation to the frequency-distribution of the DNA values of interphase cells. It is concluded that information on all of these may be needed to accurately characterize a tumour. However, for most tumours, the ploidy-level indicated by the modal interphase DNA value is in agreement with that obtained from chromosome counts. In a few tumours, the main interphase DNA mode is in the tetraploid or hypertetraploid region and there is also a small near-diploid mode. Two such cases are illustrated, and it is concluded from the data provided that (i) other regions of the tumour may show a reversed picture, the main mode being in the diploid region although there is still a substantial tetraploid mode, and (ii) the majority of metaphases in such tumours may sometimes have near-diploid chromosome numbers. The significance of these findings is discussed. Data on nuclear size obtained on histological sections of carcinoma of the cervix suggest that usually there is consistency of ploidy in different regions of a given tumour; where a section shows variation, other sections from different regions usually show similar variation.