This paper describes an image analysis technique for the assessment of changes in chromatin organization in ultrathin tissue sections. Transmission electron micrographs of tissue sections were analyzed by a SEM-IPS image processing unit (Kontron) at a total magnification of 7,500. The boundaries of each nucleus and each nucleolus were defined interactively, and the grey level threshold between heterochromatin and euchromatin was determined. The grey level distribution and the total area of each nucleus was measured. In addition, the total area, number, and individual areas of heterochromatin particles and nuclei were measured. Based on these measurements, a number of different variables have been defined, and several of these have proved to discriminate between normal and malignant cells.