A METHOD of obtaining large numbers of fixed cells in metaphase arrested by colchicine, from a population of cells growing on a glass surface, by a method dependent on the attenuated adhesion of the cells to the surface has been described1. A modification of this method was suggested to obtain a viable population of synchronous cells. This was demonstrated2 and a method for obtaining large numbers of HeLa cells of which 80 per cent to 95 per cent showed mitotic figures has been described3. The decrease in attachment of the mitotic cells was ascribed to a decrease in the area over which the adhesion occurred1. It is, however, possible that there is a change in the intrinsic strength of the cell-glass adhesions arising from an alteration in the physical properties of the surface of the cells which round up during mitosis. One of the most easily determined of these properties is the ξ-potential arising from the cell surface charge, and accordingly we have made determinations of the electrophoretic mobility of parasynchronous populations of HeLa cells at various stages in the cell cycle.