The kinetic behaviour of subtilopeptidase A was investigated. The enzyme was obtained from a local isolate of B. subtilis PR-70. The rate of enzyme catalyzed conversion of substrate to product is directly proportional to the enzyme concentration, v = K(E). Michaelis constant was determined using different methods. The average of Km value is equal to 0.01615. The Vmax and Et were determined being 0.71 and 1.467, respectively, using KUNITZ's casein digestion method. The enzymeconcentration involved in the reaction system is equal to 97.8%. A trial to calculate the molecular weight and the number of active groups were discussed.