A new approach to immobilize mixed culture of hydrogen-producing bacteria was examined by growing on granular activated carbon in an anaerobic fluidized bed reactor. Production of hydrogen by the immobilized culture was assessed at a consistent pH of 4.0 and at a temperature of 37 degrees C. The system was operated by shortening the hydraulic retention time (HRT) from 4 to 0.5h at 10 g/L influent strength, or by increasing the influent concentration of glucose from 10 to 30 g/L at 1 h HRT. The biogas produced was composed of H-2, CO2 and free of CH4. Hydrogen composition decreased from 61% to 57% with the reduction in HRT, while it stabilized at 59% as feed strength varied. The dissolved metabolite products were acetate and butyrate, with smaller quantities of other volatile fatty acids and alcohols. The hydrogen production rate and specific hydrogen production rate were linearly correlated to the effective organic loading rate, which was calculated on the basis of organic loading and glucose conversion rate, giving the respective maximum rates of 2.36 L/L h and 4.34 mmol-H-2/g-VSS h. The attached biofilm concentration was retained as high as 21.5 g/L. It is concluded that a substantial quantity of retained biomass would enable the reactor to run at the high organic loading rates and thus enhance the production rates of hydrogen gas. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.