Enhancement of biohydrogen production via anaerobic digestion from Taihu cyanobacteria (blue algae) after acid stress on anaerobic sludge, and the enzymatic characterization of the acid tolerance response (ATR) during the enhanced biohydrogen production process were investigated in this study. Comparing to those of the control, biohydrogen accumulation and hydrogen content increased by 1.9 and 1.7 times, when 12.5 and 7.5 g/L of acid stress on anaerobic sludge were performed respectively. Other than that, activities of hydrolytic enzymes, such as B-glucosidase, BAA-proteolytic enzyme and phosphatase were all improved during the enhanced biohydrogen process after appropriate acid stress. Significantly, activity of glutamate decarboxylase (GAD), the main microbial ATR stimulated by excessive acids, was increased consistently with the biohydrogen accumulation. Therefore, acid stress might be a practical approach to improving the biochemical traits of the anaerobic sludge. In turn, improved hydrolysis of organic substances would help the anaerobic sludge better survive excessive organic acids, and then enhance biohydrogen production from Taihu cyanobacteria. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.