Certain changes in human carcinoma A-431 are found by scanning electron microscopy. The early cell response to growth factor (after 10 minutes) involves a disappearance of microvilli, an appearance of ruffles and rounded cells, along with a decrease in cell attachment to the substrate. The cell surface changes correlate with the state of cytoskeleton elements: the material stained with iron hematoxylin is accumulated in ruffle formation sites. Retractional fibrils filled with the cytoskeleton material result from a decrease in the cell area.