Cells exfoliated from the uterine exocervix from normal women at different stages of the reproductive period and from patients with invasive carcinoma were studied. Cell pellets were fixed in aldehydes and two different concentrations of OsO4, and embedded in methacrylate or Epon. Semithick sections were used for general light microscopic study and for the visualization of glycogen. Ultrathin sections were used for conventional electron microscopy, high resolution analysis of the plasma membrane, and the demonstration of glycogen and cell surface glycoconjugates by the Thiery method. Semithick sections stained with the Thiery method and viewed under the electron microscope were used for the study of surface projections. Based on the size, shape, nuclear characteristics, amount and distribution of glycogen, type of surface protrusions, density and distribution of surface glycoconjugates, and plasma membrane fine structure, the cells exfoliated from all normal uterine cervices were grouped into five cell types. It is suggested that these types correspond to cells located in the different layers of the exocervical epithelium and, consequently, represent different degrees of normal differentiation. The plasma membrane of carcinoma cells shared most of the characteristic of that of the least differentiated normal cells, indicating an early deviation of the differentiation process in carcinoma cells.