Atomic force microscopy, in contact mode, has been used to image living mammalian cells in culture at both low and high resolutions. The method is shown practical for revealing cytoskeletal features beneath the cell membrane and their restructuring during a variety of cellular activities. Among the processes that we have visualized are locomotion, tissue formation, cell division, transformation by viruses, and cell death. We show that some processes that occur well within cells can, nonetheless, be observed using the atomic force probe. At high resolution, features on the cell surface on the order of 0.5 mu m, and their changes with time, can be recorded. (C) 1997 Academic Press.