Batch experiments were performed to investigate the effect of particulate protein size on the hydrolysis of casein in anaerobic degradation. Whereas particle size did not affect the ultimate protein degradation efficiency, the hydrolysis rate coefficient increased from 0.034 to 0.298 day(-1) with a change in specific surface area from 0.01 to 0.192 m(2)/g. The maximum rate of methane production increased from 6 to 14 mL of CH4 per gram of COD added per day with the same change in specific surface area, although the ultimate volume of methane produced was approximately the same despite the change in specific surface area. A mathematical relationship between the hydrolysis rate coefficient and the specific surface area was developed, and a new hydrolysis equation was proposed and verified.