In order to improve the enzymatic saccharification of hybrid poplar sawdust, gamma irradiation pretreatment and enzymatic hydrolysis in the presence of sludge protein were investigated. The cellulose crystallinity index were significantly decreased after irradiation pretreatment, and adding sludge protein improved enzyme activity and increased the reducing sugar yield. The conditions of irradiation pretreatment and enzymatic hydrolysis in the presence of sludge protein were systematically examined. The maximum reducing sugar yield was 519 mg/g under an irradiation dose of 300 kGy, a sludge protein dosage of 2 mg/mL, an enzymatic hydrolysis temperature of 45 degrees C, an enzymatic hydrolysis time of 84 h, and a 90 FPU/g enzyme loading. This work indicated that the combined method of gamma irradiation pretreatment and enzymatic hydrolysis in the presence of sludge protein was a promising potential for the saccharification of hybrid poplar sawdust. (C) 2016 Elsevier Ltd. All rights reserved.