Thermodynamics of free and immobilized subtilopeptidase was studied over the temperature range from 30 degrees to 80 degrees C and from 30 degrees to 90 degrees C, respectively. The apparent optimum temperature of free enzyme was 50 degrees C and of the immobilized enzyme 60 degrees C. The heat of the reaction (delta E), accompanied by the process, was calculated from the slope of the Arrhenius equation to be 56 770 cal/mole for free enzyme and 36 640 cal/mole for immobilized enzyme. The standard free energy change (delta G) and the standard entropy change (delta S) were found to be + 241 cal/mole and + 186.6 cal/mole/degree at 30 degrees C for free enzyme and + 142 cal/mole and 120.5 cal/mole/degree at 30 degrees C for immobilized enzyme. The values of these thermodynamic quantities at other temperatures were also summarized. Studying the reaction order of heat inactivation showed that free and immobilized enzyme followed a first order reaction. The energy of inactivation was found to be 18 900 cal/mole and 24 400 cal/mole for free and immobilized enzyme, respectively.