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The effect of salts (KI, KBr, NaCl, KCl, KF, phosphate, and Na2SO4) on the stability of beta-galactosidase in aqueous solution was studied from the aspect of changes in water mobility. At salt concentrations up to 200 mM, the inactivation rate of beta-galactosidase in all the salt solutions studied increased with increasing salt concentration. At higher concentrations, those salts which had little effect on the spin-lattice relaxation time, T1, of water (KI, KBr, and KCl) continued to increase the inactivation rate of beta-galactosidase with increasing concentration, while those salts which decreased the T1 of water (KF, phosphate, and Na2SO4) decreased the inactivation rate. It appeared that the decrease in water mobility caused by KF, phosphate, and Na2SO4 resulted in stabilization of beta-galactosidase. The results indicate that water mobility is an important factor in the denaturation rate of proteins.