We have studied the enzymatic hydrolysis of whey proteins at pH 8 and 50 degrees C with two proteases of bacterial origin, MKC Protease 660 L and Alcalasa 0.6 L, and one of animal origin, PEM 2500 S. Our results show that a greater degree of hydrolysis is achieved under the same experimental conditions with the bacterial proteases than with the animal one. In our interpretation of the results we propose a mechanism in which the hydrolytic reaction is a zero-order one for the substrate, and the enzyme denaturalizes simultaneously via a second-order kinetic process due to free enzyme attacking enzyme bound to the substrate. Our results also indicate that there is an irreversible serine-protease inhibitor in whey proteins.