Thermodynamic stability of an alkaline protease from a new alkaliphilic Nesterenkonia sp. AL-20, was investigated and compared with that of Subtilisin Carlsberg. The amount of calcium bound to the AL-20 protease was determined to be only about 0.14 mol/mol of protease. Differential scanning calorimetry scan of the enzyme at increasing temperature showed the denaturation of the enzyme to be a two-state process with melting temperature, T-m of about 74 degreesC at pH 10.0, which was unaltered upon addition of calcium as well as after treatment with chelating agents. The thermodynamic parameters were nearly the same over a pH range of 7.0-10.0. T-m was reduced to 69.7 degreesC at pH 6.0 and 72 degreesC at pH 11.0. The secondary structure of the protease was unaffected during storage at 50 degreesC, even in the presence of 1% SDS as observed by circular dichroism. The protease activity was extremely stable in the presence of hydrogen peroxide and various sequestering agents used in detergents.