By adding a small amount of L-cysteine (0.1-1.0 mM) to nutrient solution, the dark fermentative hydrogen production using anaerobic mixed culture at two mesophilic temperatures, 35 degrees C and 30 degrees C, were largely enhanced. Compared to those of the blank tests, the hydrogen productivities and substrate degradation efficiencies of the tests with L-cysteine were all increased. Moreover, the maximum hydrogen production rates were increased to 1.5-2.9 times. In particular, the hydrogen yields peaked at 3.10 mol H(2)/mol sucrose with addition of 0.6 mM L-cysteine at 35 degrees C and at 3.28 mol H(2)/mol sucrose with addition of 0.1 mM L-cysteine at 30 degrees C, which were 15.2% and 70% higher than those of the corresponding blank ones, respectively. The results show that L-cysteine can be used as a low-cost and highly efficient bioactive agent to increase dark fermentative hydrogen production. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.