Determinations of electrophoretic mobility were made on Landschutz ascites carcinoma cells and mouse leucocytes suspended in a series of solutions oโฌ differing hypotonicity. The curves relating volume to the reciprocal of tonicity indicated that both types of cells conform fairly well to the Van't Hoff-Mariotte Law from a relative tonicity of about two down to 0.325. Beyond this point, leucocyte volumes tended to taper off to a plateau whereas ascites cell volumes first showed an increase in slope and then leveled off. Reversal of hypotonically swollen cells to isotonicity brought about a shrinking of the cells back to their original volume. These expanded and contracted cells were shown to retain other of their normal properties such as impermeability to vital dyes and for ascites, the ability to proliferate upon transplantation. It was concluded from these observations, that throughout their extensive expansion in hypotonic media, the cells remained essentially viable and intact. The possible effects of increase in cell volume on surface area changes were discussed. There was no change in the electrophoretic mobility of the cells in the entire range of hypotonic solutions tested. Enlargement of cell volume without change in mobility may have been produced by an expansion or unfolding of the ultramicroscopic crenations of the membrane and a corresponding exposure of similar surface to the electrokinetic plane of shear.