1. L-Alanine dehydrogenase (L-alanine:NAD+ oxidoreductase (deaminating), EC 1.4.1.1) was purified about 500-fold from Halobacterium salinarium. 2. The enzyme appears to be homogeneous in polyacrylamide gel electrophoresis. The apparent molecular weight is about 60 000. 3. Activity and stability of the enzyme are largely affected by different salts. Full activity of the NADH-dependent reductive amination of pyruvate occurs at 4.3 M NaCl. This activation can be achieved also by KCl and several other salts instead of NaCl. 4. The NAD+-dependent oxidative deamination of L-alanine occurs only in the presence of high concentrations of KCl. This reaction is not stimulated by NaCl. The Km values for the substrates NADH, pyruvate and NH+4 are also salt dependent. 5. The thermal stability of the enzyme is considerably higher in the presence of high concentrations of NaCl than in the presence of KCl. 6. The enzyme is completely inactivated by the removal of salt. Full reactivation is achieved by addition of salt in the presence of 2-mercaptoethanol. Inactivation proceeds about ten times faster than reactivation. The inactivation after the withdrawal of salt and the reactivation following the readdition of salt show a characteristic hysteresis loop.