In this study, the effects of endogenous amylase, endogenous protease and combined amylase/protease pretreatment of sludge were studied to enhance the efficiency of sludge anaerobic digestion. These enzymes were obtained from bacterial fermentation and bacteria were separated from the sludge. All treatments improved sludge solubilization and acidification but had little influence on the floc sizes. In terms of sludge solubilization and acidification amylase was better than protease or mixed enzyme. After 7 h endogenous amylase treatment, the supernatant soluble chemical oxygen demand and volatile fatty acids concentration increased by 78.2% and 129.6%, respectively. But, in terms of anaerobic biodegradability, the best result was obtained with combined enzyme treatment, biogas production increased by 23.1% compared to the control after 11 days of anaerobic digestion. Scanning electron micrographs observation and particle size analysis revealed that the most important mechanism for the enzyme treatment of sludge might be solubilization of extracellular polymeric substances. (C) 2013 Elsevier Ltd. All rights reserved.