The cryoprotective effect of monosodium glutamate (MSG) and lysine hydrogenchloride (Lys-HCl) on lactate dehydrogenase was investigated in comparison with the effect of sodium chloride. The cryoprotective effects of MSG and Lys-HCl as solutes seem related to their zwitterionic properties. Thus their pH buffer action and physicochemical modifying actions on the freezing process of water were examined. No pH buffer action, as estimated from color changes of a universal pH indicator, was observed for either 0.2 M MSG or Lys-HCl. Physicochemical modifying actions, such as freezing-point depression and supercooling, were demonstrated on all the additive solutions. The pH buffer action of the additives was probably not related to cryoprotective effect, while the termination of the supercooling, more precisely the formation of ice, was found closely related.