Observations have been made on the response, in vitro, of cultured and freshly dissociated cells to mechanical deformation. Large numbers of individual cells were studied by means of a special culture chamber bounded by two parallel glass coverslips whose spacing could be reduced from 140 to 2 microns in steps of roughly 0.5 micron. The degree of deformation required for herniation of the cell surface was measured. These measurements lead to the definition of a statistical index characteristic of the extensibility of cell surfaces. This index has been shown to be distinctive for several types of cells; to alter with certain stages of embryonic development; and to be stable with respect to the culturing of cells and certain alterations in the method of cell culture.