The object of this study was to measure some of the differences between normal squamous epithelial cells and cells from invasive squamous carcinoma of the uterine cervix. A total of 107 patients were studied; only those specimens which when assessed by a histopathologist were thought to show classic normal features or undoubted invasive carcinoma were included in the quantitative analysis. In addition, any specimens which at the electron microscope level, showed faulty sampling or preparation were discarded, leaving us with 16 carcinoma and 15 normal specimens for detailed study.The nuclei of tumour cells had a greater area than those of normal cells; histograms of the size distribution of nuclei showed a distinctly different pattern in the 2 groups. Tumour cells had fewer ribosomes in each cubic micron of cytoplasm than had the normal cells and showed a reduction in the amount of intercellular space; in addition, the malignant cells had a smaller surface density and fewer tonofibrils in their cytoplasm. Some tumour cells had a smaller percentage of cell membrane specialized as desmosomes than the corresponding normal cells but all tumour cells had desmosomes of shorter length than normal.Discriminatory analysis, carried out with the help of a computer, allowed all of these variables to be assessed with respect to each other in order to arrive at a numerical score for each specimen. When expressed graphically, these scores showed that the populations of normal and carcinomatous cells fell into 2 separate groups. The significance of these results is discussed.