Biological degradation of natural estrogens was investigated by separately spiking 17 beta-estradiol (E2) and estrone (E1) into activated sludge reactors operated in a semi-continuous flow mode under aerobic conditions. By conducting runs for simultaneous addition of glucose over the designated initial concentrations of 0, 10, 30, 50 and 100 mg l(-1), respectively, the likely effect of the easily biodegradable organic constituent on the degradation rate of the natural estrogens was also studied. The spiked E2 disappeared in a manner much faster than E1; and its disappearance occurred through elimination of its metabolic byproduct of E1. The apparent first-order disappearance rate of E2 and E1 fell in the ranges of 0.84-4.31 h(-1) and 0.15-0.84 h(-1), respectively when spiked separately into respective reactors. A general trend of decreases in the disappearance rate with increases of the initial glucose concentration was revealed. The presence of glucose was also found capable of lowering the conversion ratio from E2 to E1. (C) 2007 Elsevier Ltd. All rights reserved.