Solutions of NaCl or KCl at a concentration of 0.5 M collapse the new-laid egg of the house cricket, Acheta domesticus (L.), and these effects are prevented by 0.01 M CaCl2 and to a lesser extent by 0.01 M MgCl2. Collapse of the egg by the NaCl or KCl solutions can be reversed by a subsequent addition of 0.01 M CaCl2; return to a NaCl or KCl solution leads again to collapse. This antagonism between monovalent and divalent cations could explain certain properties of the living cell.