A scanning electron microscope study of the Swiss 3T3 cell surface topography was made in association with changes in proliferative activity. On the phase of logarithmic growth the cell shape is similar to that of an actively migrating fibroblast. When the growth promoting factors are depleted, the cells are seen spread and forming numerous cell-to-cell contacts. The stimulation of cell proliferation induced by a 10% serum or by epidermal growth factor is associated with a disturbance of cell interaction because of the centripetal cell contraction.