With the aim of efficiently utilizing livestock waste as a protein resource, we studied their enzymatic hydrolysis. Particles of a mixture of horn and hoof from cow and buffalo with a diameter of less than 250 mu m were selected for use as substrate, and a proteinase from Bacillus subtilis, selected from among various commercial proteolytic enzymes was used. For enzymatic hydrolysis of the substrate, heat treatment was absolutely necessary prior to digestion and the conditions for the enzymatic reaction were determined to be as follows : reaction time, 30 to 60 min; pH, 8.3; temperature, 50 degrees C; weight ratio of substrate to enzyme, 1:0.05; and concentration of substrate, 62.5 g/l. Even when a membrane reactor was used, the amount of enzyme required could not be reduced. However, repeated-batch reaction allowed us to reduce the added amount of enzyme. The enzyme was added only for the first round of the reaction in four rounds of repeated-batch reaction. As a result of this process, the weight ratio of substrate to the enzyme was reduced to 1:0.023 from 1:0.05 at a final substrate concentration of 61 g/l.