The lactic dehydrogenase ofHalobacterium salinariumis unstable and inactive at low solute concentrations. The irreversible loss of activity at low solute concentrations is a first-order reaction, the rate of which decreases with the second or third power of the salt concentration. The enzyme is protected to varying degrees by a variety of inorganic salts and by glycerol or glucose. Potassium chloride and glycerol also protect the enzyme against inactivation by urea.The enzyme is most active in the presence of potassium chloride, but several other salts are effective to varying degrees as activators. The activation is of the third or fourth order with respect to sodium or potassium chloride concentration. Potassium chloride decreases the affinity of the enzyme for its substrate. Glycerol does not activate the enzyme, but increases its activity in the presence of potassium chloride.It is suggested that this enzyme differs from enzymes of non-halophilic organisms in being less firmly held in its native catalytically active conformation. At low salt concentrations the electrostatic repulsion between ionized groups on the enzyme expands it to a form which is not catalytically active and which can readily expand further to an irreversibly denatured form.